Stage twelve, the first mountain stage in the Pyrenees, saw Cazala take his second win from a sprint between an elite group of climbers and overall favourites who finished together after the descent to Saint-Gaudens. Schroeders could not keep up with this group, which included British rider Tom Simpson, and Simpson took the overall lead, becoming the first non-mainland European in history to wear the yellow jersey. Bahamontes led over the first-category Col du Tourmalet, and the two other lower categorised climbs, to take the lead in the mountains classification.