Seven-times Tour de France winner Lance Armstrong crashed on the first stage of Spain's Vuelta Castilla y Leon on Monday and was taken to hospital with a suspected collarbone fracture, organisers said.
The American was caught up in an incident involving several riders on the 168.3-km leg between Paredes de Nava and Baltanas and appeared to be in pain as he was helped into an ambulance.
A statement on the race website (www.vueltacastillayleon.com) said the Astana rider had been taken to the University Clinic in Valladolid for checks on the suspected fracture.
The Vuelta was being closely scrutinised as it was the first and only time Armstrong was due to race alongside Spanish team mate Alberto Contador before July's Tour de France.
Contador, the world's best stage-race rider, won the Tour in 2007 and there has been speculation about which of the pair will establish themselves as Astana's number one.
A team spokesman said it was too early to say whether Armstrong would be able to ride in May's Giro d'Italia.
Monday's incident occurred with about 20-km remaining.
Spaniard Joaquin Sobrino of the Burgos Monumental team won the sprint finish.