We performed monthly examinations and serologic tests for antibody to hepatitis virus A (anti-HA) in a study of 57 heterosexual men and 102 homosexual men followed prospectively for a mean of 8.9 months and 6.1 months, respectively. The initial prevalence of anti-HA was 30 per cent in homosexual men and 12 per cent in heterosexual men P less than 0.01). The annual incidence of hepatitis A in susceptible (seronegative) homosexual men was 22 per cent, whereas no heterosexual men acquired hepatitis A. Diaries concerning sexual behavior kept by homosexual men showed that the acquisition of hepatitis A virus infection contact. Hepatitis A should be considered one of the enteric infections that appear to be sexually transmitted among homosexual men.