In 1980-1981 a study for the presence of markers of hepatitis B virus was done among 2, 946 homosexual men living in The Netherlands, mainly in Amsterdam. Serologic evidence of a past or present hepatitis B infection was found in 60.3% of this group, and the prevalence differed significantly among the different age groups. Of the total population studied, 4.8% were positive for HBsAg. This prevalence is very high in comparison to that in the general Dutch population, among whom the prevalence of hepatitis B is low, as is shown by the fact that the prevalence of HBsAg was 0.22% among a large group of Dutch first-time blood donors. Another blood sample was collected two to 20 months after the first sample from 316 male homosexuals who were initially negative for hepatitis B markers. It was found that 36 persons had seroconverted, most of them without clinical manifestations of hepatitis. The annual attack rate of hepatitis B in this group was calculated as 27.6% and differed significantly among the different age groups.