His biggest victory was capturing the 1982 Vuelta a España, a Grand Tour stage race, and he is the inaugural and record three-time winner of the Clásica de San Sebastián (1981, 1982, 1987), which is now considered a one-day classic.

Lejarreta rode very well in the 1982 Vuelta, but initially finished 2nd to Ángel Arroyo while coming in just eighteen seconds ahead of Michel Pollentier.

During the Lagos de Covadonga stage he escaped from the group of favorites made up of Fernandez, Pino, Hinault, Kuiper and others and soloed to victory, but it was not enough to reclaim the lead.

During the 1990 edition he found himself among the group of favorites as Delgado, Breukink and LeMond were battling the surprising Claudio Chiappucci for the Yellow Jersey.

With the available technology not giving instant times, results and standings as it does today, it was not as easy to know what was going on from within the race and Lejarreta didn't find out until he came to a stop and was told by his team that he had won the stage.