The Scarlets were the only Welsh region to make it past the Pool stages; the Celtic Warriors were unlucky not to make it through to the last eight, having finished second in Pool 6, just missing out on one of the two highest points-scoring runners-up qualifying places available. The Dragons and Blues, both with two wins each, and the Ospreys, with just one victory, all went out at the end of the Pool stages. The Scarlets were the sole Welsh representatives in the elimination stages of the competition.

The Newport Gwent Dragons faced an almighty battle in Pool 1 against opponents Stade Francais, Ulster Rugby and Leicester Tigers. In their first round game the Dragons overcame the Irish in a 24-15 victory at Rodney Parade. The Dragons’ forward power and two tries to nil advantage, courtesy of Ben Breeze and Percy Montgomery, swung the game and produced an impressive start to their Heineken Cup campaign. A week later the Dragons travelled to Leicester to take on the Tigers with the recently crowned Rugby World Cup winning captain Martin Johnson amongst their ranks. Having lost to Stade Francais the previous weekend the Tigers sought revenge with a 34-3 mauling of the Gwent region, earning a bonus point in the process.

The third round of the competition saw the Dragons return to winning ways with a victory over joint Pool leaders Stade Francais. However, the second meeting of the two teams in Paris a week later saw the Dragons unable to maintain their form as they endured a crushing 37-0 defeat.

Due to this ill-disciplined showing, in which they gained three yellow cards, the Dragons slipped to the bottom of the Pool 1 table, though qualification was still a possibility.

The fifth round saw the Dragons stage their home fixture with Leicester. The Tigers secured a 20-26 win in which they ended the Dragons' hopes of qualification and set themselves up for a pool decider with the French giants. Though the Dragons had won a bonus point it was no consolation for enduring their first loss of the season at Rodney Parade.

Round Six brought a further defeat, by Ulster, who maintained their 100% home record in the competition to put the Dragons out of the competition.

The Neath-Swansea Ospreys opened their Heineken Cup campaign unsuccessfully away from home. Pitted against the Leeds Tykes the visitors incurred a 29-20 defeat despite a second-half fightback; a Leeds penalty in injury time also snatched away a losing bonus point from the Ospreys. The second round saw the Ospreys host Celtic League rivals Edinburgh Rugby at St Helen’s. In appalling weather conditions the Scottish scored four tries to gain the bonus point and attain the top spot of Pool 2 after two rounds. The Ospreys next travelled to French counterparts Toulouse. In a comprehensive display on a sodden pitch, home side Toulouse ran in four tries to gain a bonus point over a Welsh region that executed poor decisions and error-strewn play. The only points the Ospreys secured in the 29-6 defeat came from the boot of Gavin Henson.

The Ospreys incurred their fourth successive defeat in their second meeting with defending Heineken Cup Champions Toulouse. Vincent Clerc scored a hat-trick of tries, though Henson pulled one back for the home fans to cheer, but as the French side notched up four scores they racked up another bonus point in their impressive run.

The Ospreys travelled to Murrayfield for Round Five and their second encounter with Edinburgh. The Ospreys’ 33-15 defeat meant that Edinburgh became the first Scottish side to make it into the quarter-final stages of the Heineken Cup competition. Henson gained all of the Osprey points in the four-try showing by the Scots, which rooted the Welsh side to the foot of the table with nil points.

The Ospreys finished their 2003–2004 campaign on a high note with a consolatory win over Leeds Tykes at home. Avoiding a pool whitewash the Ospreys secured a try through Andy Williams with Henson adding the remainder of the points in the 10-3 victory.

As Gordon Ross obtained a penalty the Tykes achieved a bonus point, leaving the Welsh region firmly fixed at the foot of the table with four points.

The Cardiff Blues joined Biarritz, Sale Sharks and Leinster in Pool 3 of the Heineken Cup. Their first match against the Sharks saw the Blues lose out in a 26-24 away defeat. The determined Welsh region matched the two tries of Sale Sharks with efforts from Tom Shanklin and Nathan Thomas but came unstuck due to a late Charlie Hodgson drop goal. The losing bonus point was a small consolation for the Welsh region. The second round of Cardiff’s campaign saw them host Leinster at home in which they were again narrowly defeated. A Shanklin try equalled the earlier try-scoring efforts of the Irish visitors but man-of-the-match Keith Gleeson added another five-pointer for the visitors which proved the decisive score in the 19-24 victory. Another losing bonus point kept the Welsh side in contention.

Round Three in Pool 3 saw the Blues incur their third successive defeat in the Cup campaign. Away at Biarritz Olympique, Cardiff made the brighter start with a Craig Morgan try after just thirty seconds. However, a Biarritz fightback saw the team obtain four tries and the resultant bonus point after indiscipline meant the sin-binning of two Cardiff Blues stalwarts, Rob Appleyard and Craig Quinnell.

Seven days later in the return leg the Blues enacted revenge on the French side with a narrow 21-20 victory at home. Ending their ten match European losing streak, the Blues clung onto their one point lead after Dimitri Yachvili missed a simple try conversion a minute from time. This first win in the competition built on their two points in the Pool 3 table.

The Blues next travelled to Ireland to take on the Leinster Lions but came up against a determined home side that ground out a 20-17 win. The Welsh region was 17-3 behind at one stage in the match but levelled the scores following tries from Jim Brownrigg and Craig Quinnell. Leinster pulled away four minutes from time with a penalty to break the deadlock and to move closer to the knock-out stages of the tournament.

In the Blues' final match of the campaign they secured a win over Sale Sharks following their opening match defeat to the Zurich Premiership side. The 22-7 victory ended the Blues’ competition on a high note after a frustrating 2003–2004 tournament.

The Celtic Warriors opened their Heineken Cup campaign with a bonus point win over the unfancied Italian outfit Calvisano. The Warriors had to overcome a shock 22-21 half-time deficit but two late Ceri Sweeney penalties secured the game for the region and denied the Italian side a losing bonus point.

The second round match pitted the Warriors away against Perpignan. The French side narrowly secured a 26-19 bonus point victory due to an injury time try courtesy of Rimas Alvarez-Kairelis but could not prevent the region from grasping a losing bonus point as the conversion went amiss. Round Three witnessed an impressive Celtic Warriors win at the home of Zurich Premiership giants London Wasps in what was probably Wales's greatest high in the tournament of the 2003-2004 season.

The Warriors pulled out a 9-14 win over one of the Heineken Cup favourites due to fierce defence, tough tackling and a fantastic try from Tongan Aisea Havili that proved the match winner. Just days later the Warriors hosted Wasps at the Brewery Field and suffered a 12-17 defeat at the hands of the eventual 2003–2004 Heineken Cup Champions.

A week later the Warriors hosted another Brewery Field encounter in which they triumphed over Perpignan in a 16-15 dramatic finale. The Warriors were awarded an 80th minute penalty try after sustained pressure on the French line; Neil Jenkins secured the match-winning conversion to keep the Warriors’ Cup dreams alive.

In the final round the Celtic Warriors secured a narrow 26-28 bonus point victory away at Calvisano only to be knocked out of the Heineken Cup competition. Due to the London Wasps’ comfortable bonus point win over Perpignan, the Warriors were denied the top spot. Finishing in second place with 20 points, the Warriors points tally was unfortunately not high enough for the region to finish as a highest-placed runner up. Despite exiting at the pool stage, the inaugural competition played as the Celtic Warriors would be hailed as a successful one.

Llanelli Scarlets were the sole Welsh region to make the last eight of the 2003–2004 Heineken Cup Championship. Drawn against Northampton Saints, Agen and Border Reivers in Pool 4, the Scarlets met the Saints at Stradey Park in their campaign opener. Scott Quinnell bagged an early try to secure a 14-9 victory for the organised Scarlets side, though Northampton did manage to obtain a losing bonus point due to a late penalty from Paul Grayson. The Scarlets pulled out an emphatic second round victory at Netherdale against the Borders. In the 10-41 bonus point win Barry Davies secured a brace of tries along with one each for Scott Quinnell, Dwayne Peel and Garan Evans whilst Stephen Jones joined Diego Domiguez and Ronan O’Gara in having achieved five hundred points in the Heineken Cup competition after kicking sixteen points in the match. The Llanelli side hosted a brave Agen at Stradey and were only capable of a slender 19-15 win, ensuring that the French side attained a losing bonus point. Agen avenged this narrow loss the following week when they hosted the Scarlets, inflicting the first Scarlet defeat of the competition thus far. The Scarlets went down 22-15 in difficult weather conditions to the side making their Heineken Cup debut but due to the losing bonus point achieved remained level on points with pool leaders Northampton.

Round 5 saw the home leg tie with Border Reivers in which the Scarlets demolished the Scottish side in an eight-try flattening. Scott Quinnell continued his try-scoring form by bagging a brace and creating a third. As the Borders’ defence weakened the Scarlets tally grew to 53-7 at the close play.

The Scottish debutants remained pointless in the competition after this hammering by the Welsh region. Having overcome the Borders, the Scarlets set up a winner-takes-all Pool decider with Northampton the following week.

The Scarlets out-scored their Pool rivals in a 9-18 thriller and by two tries to nil; Barry Davies scoring a brilliant try after claiming the ball on the half-way line and racing deep into Saints’ territory.

The Scarlets dominated the last half an hour of the game to obtain first place in Pool 4, Stephen Jones was on hand to ensure Davies's try was converted. Two late substitutions from the Saints did little to effect the outcome of the game and the Scarlets secured Welsh regional representation in the last eight of the competition.

The Scarlets hosted Biarritz Olympique at Stradey Park in a quarter-final that dashed Welsh aspirations. In front of a capacity crowd the French made the brighter start and secured the first try yet with the scores at the interval resting at 5-3 the Scarlets remained in contention.

Despite the depletion of a man in the sin-bin, Biarritz scored another try on the restart but this was soon followed by a score in kind by Scarlet stalwart Stephen Jones, who secured all of the home side's points of the evening. After regaining their composure Biarritz soon after began to dominate with Nicolas Brusque securing his second try of the game.

Philippe Bernat-Salles secured a further try for the visitors during injury time to seal the Biarritz victory and the Scarlets ejection from the 2003–2004 Heineken Cup competition.

Though the Scarlets' exit had dashed the hopes of Welsh regions progressing further into the elimination stages of the tournament, there was one note of Welsh interest that followed all the way through to the final at Twickenham in the form of former Wales scrum half and skipper, Rob Howley.

Playing for the season's Pool 6 winners, London Wasps, Howley refused to give up on a long range grubber kick of his own making late in the game. He kicked a return ball from a Toulouse 22m restart to the far corner of their try line and chased the ball down in the dying minutes of the match.

Toulouse had drawn level at 20-20 minutes earlier and extra time was looming. Clement Poitrenaud waited ponderously to either usher the ball into touch or place it down behind his own line to set up a 22m restart.

Howley, however, had other ideas and pounced on the ball to score a dramatic, remarkable late opportunist try, earning himself near legendary status and win the Heineken Cup for Wasps in his last ever professional rugby appearance.