The event started and ended in Paris, covering 3,427.5 km (2129.75 mi), proceeding clockwise in twenty stages around France, including six major mountain stages. Due to the centennial celebration, this edition of the tour was raced entirely in France and did not enter neighboring countries.
Of the 198 riders the favorite was again Armstrong, aiming for a record equalling fifth win. Before the race, it was believed that his main rivals would include Iban Mayo, Aitor González, Tyler Hamilton, Ivan Basso, Gilberto Simoni, Jan Ullrich, and Joseba Beloki but Armstrong was odds-on favorite. Though he did go on to win the race, it is statistically, and by Armstrong's own admission, his weakest Tour from his seven-year period of dominance over the race.
The team selection was done in three rounds: in November 2002, the fourteen highest-ranking UCI teams would automatically qualify; four wildcards were given in January 2003, and four wildcards in mid-May.
In the first round, the Coast team had been selected to compete, and in January 2003 they signed Jan Ullrich. Financial problems then almost prevented the team from starting, but after Bianchi stepped in as a new sponsor, Team Bianchi was allowed to take the place of Team Coast.
The Tour proved to be one more hotly contested than the previous years. Tyler Hamilton and Levi Leipheimer were involved in a crash early in the Tour. Leipheimer dropped out, Hamilton continued and got fourth place in the end while riding with a broken collarbone.
In the Alps, Gilberto Simoni and Stefano Garzelli, first and second in the Giro d'Italia earlier the same year, could not keep up with Lance Armstrong and the other favorites. The same held for last year's number 4, Santiago Botero. Joseba Beloki could, and was in second-place overall (just 40 seconds behind Armstrong) when he crashed on a fast descent from the Cote de La Rochette, shortly after passing the Col de Manse into Gap. The crash was a result of a locked brake, caused by a lack of traction from melting tar on the road, which led to the tyre coming off the rim. Beloki broke his right femur, elbow and wrist, and had to leave the Tour. Armstrong made a detour through the field beside the road to avoid the fallen Beloki. Armstrong was in yellow, but Jan Ullrich won the first time trial by one minute and 36 seconds. He and Alexander Vinokourov were both within very short distance from Armstrong.
On 24 August 2012, the United States Anti-Doping Agency announced that they had disqualified Armstrong from all his results since 1998, including his victory in the 2003 Tour de France. The Union Cycliste Internationale, responsible for the international cycling, confirmed this verdict on 22 October 2012.