Eleven cases of hepatitis B virus (HBV) infection (5 cases of acute hepatitis B and 6 of subclinical infection) were detected among 65 members of our university's American football team during a period of 19 months. All tested positive for antibody to the hepatitis B surface antigen (HBsAg) or core antigen (HBcAg). The incidence of HBV infection among team members (20.4%) was significantly higher (P<.001) than among students who tested positive for andibody to HBsAg throughout the university (1.8%). We also detected a single carrier of hepatitis B e antigen (HBeAg) on the team. Analysis of HBsAg subtypes in 3 of 5 players with acute hepatitis B indicated that their subtype (adr) was identical to that of the HBeAg carrier. All players with acute hepatitis B belonged to the same training group, which also included the HBeAg carrier. Our analysis suggests that horizontal transmission of HBV can occur even in a sports team, probably due to contact with open wounds during training.