Helie injured his head, his team said. He landed hard and was picked up from the icy Stelvio course by helicopter.
Sullivan was the first skier out of the gate and lost control while landing a jump midway down. He eventually got up and skied down under his own power. He was brought by helicopter to a hospital in Sondalo, where he was being checked for head injuries. He appeared to have no broken bones or other injuries.
Organizers said arrangements were being made to airlift both skiers to a hospital in Innsbruck, Austria, that specializes in head injuries.
Sullivan was conscious and alert but will remain hospitalized for observation, U.S. Ski Team medical director Kyle Wilkens said.
Sullivan is almost certain to miss Wednesday's race, another blow to a U.S. squad already missing two other downhillers. Andrew Weibrecht dislocated his left shoulder during training last week. Erik Fisher has a bruised knee after hooking a downhill gate in Val Gardena this month.