The prevalence and incidence of infection by the human immunodeficiency virus (HIV) has been under study in a cohort of 1,034 single men recruited by area probability sampling from a six kilometer square area of San Francisco where the epidemic of acquired immunodeficiency syndrome (AIDS) has been most severe. Prevalence of infection among homosexual/bisexual study subjects increased from an estimated 22.8 per cent during the last half of 1982 to 48.6 per cent during the period July through December 1984. During three subsequent six-month periods, prevalence remained stable at approximately 50 per cent. Annual infection rates, measured by seroconversion among seronegative study subjects, decreased from an estimated 18.4 per cent per year from 1982 to 1984, to 5.4 and 3.1 per cent during the first and second halves of 1985, and to 4.2 per cent during the first six months of 1986. These declines were associated with reductions of 60 per cent or more in the prevalence of high-risk sexual practices associated with both acquiring and disseminating infection by the human immunodeficiency virus.