To assess the incidence and prevalence of delta hepatitis in homosexual men, we tested serum specimens for delta markers in participants in two previous studies: a hepatitis B vaccine trial among homosexual men conducted in the early 1980's and the Centers for Disease Control sentinel counties hepatitis study for 1983-1984. In the vaccine trial, men found to be hepatitis B surface antigen positive at the time of enrollment and those men who had serologic evidence of new hepatitis B virus infection during follow-up were tested. In the sentinel counties study that determined risk factors for viral hepatitis in reported cases, all homosexual men with acute and chronic hepatitis B virus infections were tested for delta markers. Specimens were tested for delta antigen and IgM and total delta antibody. In seven different cities, among 321 men found to be HBsAg positive at the time of screening, eight (2%) were positive for any delta marker. Among 290 men with new hepatitis B virus infections during follow-up, three (two coinfections, one superinfection) had serologic evidence of delta hepatitis. In the sentinel counties study, 0/63 acute hepatitis B virus infections in homosexual men were associated with delta hepatitis. This study indicates that the delta agent is an infrequent cause of viral hepatitis in homosexual men in the United States.