Lance Armstrong crashed on the first stage of Spain's Vuelta Castilla y Leon cycling tour and appeared to be in pain as he was helped into an ambulance.
The American seven-times Tour de France winner was caught up in an incident involving a number of riders on the 168km course between Paredes de Nava and Baltanas.
A spokesman for Armstrong's Astana team says he's probably hurt his collarbone or shoulder and has been taken to hospital for checks.
The event was being closely scrutinised as it was the first and only time Armstrong was due to race with Spanish team mate Alberto Contador before July's Tour de France.
Contador, the world's best stage-race rider, won the Tour in 2007.