This season saw the Celtic League reformatted from two pools with a grand play-off final between each pool winner to decide an overall Champion to a more traditional round-robin style league, with the 12 sides playing each other on both a home and away basis. The change in format occured predominantly to incorporate the new regionalised structure at the elite end of Welsh rugby and the subsequent establishment of five new regional sides at the professional end of Wales's domestic and European game; the Cardiff Blues, Celtic Warriors, [Newport] Gwent Dragons, Llanelli Scarlets and Neath-Swansea Ospreys. The Welsh regions were joined by Edinburgh Gunners, the Border Reivers and Glasgow Warriors representing Scotland, and the four Irish provinces of Connacht, Leinster, Munster and Ulster.

The Celtic League now employed an altered points system, with four league points for a straight win and one league point for a draw. A team losing by a margin of eight match points or more would get no league points, however, to ensure a reduction in negative tactics and to encourage attacking play to the final whistle, as an incentive a team losing within seven match points of the victor would get one bonus point. A team could also earn itself a bonus point if it managed to score four or more tries; irrespective of whether it won or lost. In practice, a team could win scoring six tries and as a result earn four league points plus the bonus point to a total of five league points, similarly, if their opponents were within seven match points and had scored four or five tries they could walk away with two league points out of the match; one losing bonus point and one league point on the basis of their tries scored.

The change in format of the Celtic League also saw the emergence of the Celtic Cup as a separate competition played on a straightforward knockout basis between the 12 Celtic League teams during the first third of the season. Taking in a first round of eight teams followed by a quarter-final in which the winning four from the first round played four teams seeded with a bye to the quarter-finals, the Celtic Cup was played over four rounds culminating in a December showpiece final.

A dramatic finish to the Celtic League season saw the Scarlets take a marginal one point lead over Ulster into a head-to-head title-decider on the final evening of matches at Stradey Park; the Dragons too, under future Wales boss Mike Ruddock, were in the hunt for the title, poised in third to take advantage should either of the top two slip up. It was a nail biting evening with the Scarlets and Ulster locked at 9-9 at the break, but, ultimately, the departing Stephen Jones gave his region the perfect parting gift by kicking 18 points and setting up a crucial try that helped the Scarlets to a 23-16 win over the Irish side.

The match rounded off a fantastic season for the Scarlets, who finished the campaign with an impressive 16 wins from their 22 matches, with one draw and just five defeats, leaving them with 76 points, four ahead of second placed Ulster. The Scarlets' season started with a tall order as their first fixture threw up a Welsh derby at Stradey Park against the Dragons; the eventual Champions rose to the challenge, defeating the Dragons 35-11. It was a winning pattern against the Welsh regions that the Scarlets would emulate throughout the season scoring six wins out of eight against their local rivals in Welsh derbies with only the Dragons and the Celtic Warriors taking the spoils from the Scarlets when they hosted them at Rodney Parade and the Brewery Field respectively.

The Scarlets won their four home derbies and also beat the Ospreys and Blues on the road and added to their opening day scalp of the Dragons at home by repeating the same feat by a bigger margin in the Celtic Cup. Edinburgh were the only Scottish side to take the spoils of victory from the Scarlets when they beat them 26-16 at Meadowbank. Ulster, who chased the Scarlets all the way to the wire and were crowned Celtic Cup Champions in December were one of three Irish provinces to defeat them, 10-9, at Ravenhill. The Scarlets' other Irish masters were Leinster who defeated them 31-20. Connacht, who would become something of a bogey team for the Scarlets during the 2003-2004 season and beyond, frustrated the home side in a high scoring 33-33 draw at Stradey Park, where the Scarlets, who looked comfortable at 27-13 up just couldn't turn the screw on the opposition.

The Newport Gwent Dragons celebrated a fruitless but ultimately successful season, finishing in third place under Coach Mike Ruddock who was announced as Steve Hansen's successor as Wales Head Coach before the season was out and with only a handful of Dragons games to go. The Dragons' pre-season marketing campaign promised that 'sparks will fly', and they did, as a battling campaign against their local rivals and Celtic cousins made them the second highest finisher of the Welsh regional sides. The Dragons finished their season with 16 wins and just 6 defeats from their 22 matches, leaving them on 72 points, level with second placed Ulster.

A thrilling final match of the season saw the Dragons go down at Leinster 56-39, including a first half which saw seven tries and 54 points in a game more reminiscent of a sevens match. Ironically, a bonus pointed win against Leinster on the final day would have seen the Dragons poach the title from under the noses of the Scarlets and Ulster given the lack of tries in their head-to-head clash.

The Dragons' Celtic League season began on a losing note as they went down 35-11 to the Scarlets away in the very first derby match between two of the newly created Welsh regions. It was a loss they would make up for six days later as they recorded their first win in the newly formatted competition taking their first derby scalp in the process against the Ospreys at Rodney Parade. In total the Dragons notched up a margin of five wins to three against Welsh sides in derby matches in the 2003-2004 season, dispensing with the Ospreys, Warriors and Scarlets at home and beating the Blues both home and away. The Dragons came unstuck away to the Scarlets, Warriors and Ospreys and suffered a double-dose of Scarlets misery as the men from Rodney Parade repeated their opening day defeat with a 40-6 loss in the first round of the Celtic Cup in a re-run of the fixture away at Stradey Park.

The Dragons' home record was impeccable with 11 wins out of 11, this was a better record than the eventual Champions who only managed 10 wins and 1 draw. In addition to losing three derby matches away from home, the Dragons also came unstuck away against Glasgow, Ulster and in the aforementioned final day thriller against Leinster.

The Celtic Warriors, born from the creation of a region incorporating Bridgend and Pontypridd as its main feeder clubs, endured a dramatic season, rotating Sardis Road and the Brewery Field as the venue for home games. The Warriors performed admirably in the Celtic League, ironically unable to reproduce their Heineken Cup form, which had seen them finish as the second highest Welsh region in Europe behind the Scarlets. The Warriors won 14 of their 22 matches, with eight losses and no draws, finishing fourth in the final Celtic League table with 65 points, and third amongst the Welsh regional sides, seven points adrift of the Dragons.

Neil Jenkins proved a pivotal part of the Warriors’ campaign finishing as the Celtic League’s highest points-scorer with 273 points in his final season of professional rugby. The Warriors' strong league position was also largely down to an impressive finish to the season, which saw the team win their last five fixtures, including a 20-3 win at Connacht in what would prove to be the Warriors’ final game.

The Warriors started their season in fine form 12-49 away to the Borders. Leinster followed for the Warriors with another impressive win for the Welsh region in their first Celtic League game at Sardis Road. The Warriors lost their first two derby matches to the Scarlets and the Ospreys, the latter a home defeat; the Ospreys going on to do the double over the Warriors as they slumped to an away defeat as well. The Warriors revenged their defeat to the Scarlets when facing them at home and it was one win a piece when they faced the Dragons across the course of the season. The pivotal set of derby victories came for the Warriors as they did the double over Cardiff Blues with the Warriors defeating the Arms Park side 30-26 at the Brewery Field in the second match where European qualification ensured it was a game of high stakes. The result of this all-attacking game relegated the Blues to fifth place amongst the Welsh regions and meant that they would miss out on the Heineken Cup berth dictated by Celtic League placings. Ironically, when the Warriors disbanded later in the summer of that year the Blues took their Heineken Cup berth.

With no permanent home base, the region alternated home fixtures between St Helen's and The Gnoll ensuring that fans of both of the main feeder clubs; Swansea and Neath, were able to experience regional rugby in the Celtic League. A 60-7 win over Borders at The Gnoll would prove to be the Ospreys’ biggest winning margin of the season, which included a hat-trick from Gavin Henson and two tries from Shane Williams. Henson scored a record 30 points in the match, and it was performances like this which helped him break into the Wales team during the 2004 summer tour under Mike Ruddock. The season would see a change of Captain as Gareth Llewellyn was replaced Scott Gibbs, before Gibbs in turn was forced into retirement by injury.

It was an inexplicable season for the Cardiff Blues who managed to rack up a massive 73 tries; easily the highest in the Celtic League, but still finished the lowest of all the Welsh regions in 6th with 54 points. Blues outside centre Jamie Robinson finished the season as the Celtic League’s leading try-scorer, notching an impressive 12 tries. Out of the five regions the Blues faired the least well in their Welsh derbies, notching up home and away losses to all regions except the Ospreys who shared the spoils at one loss and one victory a piece.

The region based at Cardiff Arms Park started the season slowly, with just three wins from their first 15 matches, all of which came at home. A high-point of the season came with a 43-6 win over the Neath-Swansea Ospreys, their only home win over another Welsh region, a match lit up by fireman Lee Abdul, who bagged four tries. Semi-professional Abdul had been unable to gain a full-time contract with the Blues, but was drafted into the squad as cover during the 2003 Rugby World Cup. Perhaps encapsulating the topsy-turvy nature of the Blues’ season, one match later Abdul later ended up sidelined with a serious injury playing at home against the Newport Gwent Dragons.

Another high-scoring victory for the Blues came in a 60-14 hammering of Munster at Cardiff Arms Park, with a first half hat-trick from Craig Morgan providing the platform for a nine-try rout of their Irish opponents. The Blues ended the season with a record of 11 wins and 11 defeats, with no draws. Despite being the only Welsh side to receive a bye past the first round of the Celtic Cup, the Blues fell at the quarter final stage, going down 33-16 at Edinburgh.

The Cardiff Blues finished in sixth place, and bottom of all the Welsh regions. However, with the Welsh sides all coming in the top half of the table occupying five of the top six places, it was a successful year for Welsh rugby under the newly created blanket of regionalism. In the close season, Welsh internationals Stephen Jones and Gareth Thomas departed for France, and the Celtic Warriors disbanded. It didn't leave much breathing room before attention began to turn towards the challenges of the following campaign, a second year of regional rugby and for the first time in the competition's history, a Welsh region, the Scarlets, defending the Celtic League title as Champions.