BACKGROUND In adulthood, most cases of acute hepatitis B virus (HBV) infection are transmitted either by sexual contact or by contaminated needles, but there are other modes of transmission. We report on three cases of HBV infection among members of a wrestling club. CASE REPORT A 19-year-old male wrestling athlete was admitted with acute hepatitis B. Five months later, 2 other men, who were members of the same wrestling club, were diagnosed with HBV infection. The full-length sequences of the HBV DNA were identical in all three cases and classified as subgenotype C2 on phylogenetic analysis. This is the most common genotype found in Japan. No history of sexual or bleeding contact with acquaintances outside the club was noted in any of these cases. This suggests horizontal transmission within the wrestling club. CONCLUSIONS The possibility of HBV transmission through bleeding wounds and sweat is a concern in contact sports such as wrestling. Hence, hepatitis B vaccination is recommended for unvaccinated contact-sports players.