Prevalence and incidence of HIV were studied in two cohorts of homosexual men in Amsterdam between 1980 and 1987. The cumulative incidence of HIV infection increased from a weighted 2.2% in 1980 to 39% in 1987. In that year the observed incidence was less than 1%. The estimated annual HIV incidence was 3% in 1981, rose to 8.8% in 1984 and decreased gradually to 0% in 1987. This decline was assumed to be linked to a strong reduction of high risk sexual behaviour among the men under study.