One hundred and thirty-three homosexual men seropositive for the antibody against human immunodeficiency virus (HIV) were enrolled in a prospective study in 1984-85. The 3-year cumulative incidences of the acquired immunodeficiency syndrome (AIDS) and AIDS-related conditions, by life-table analyses, were 18% and 34%. The cumulative incidence of immune deficiency defined as CD4 lymphocytes less than 0.5 x 10(9) l-1 was 70% at 3 years. Absence of antibodies to p24 antigen, HIV antigenaemia, CD4 lymphocytes less than 0.3 x 10 l-1 and elevated serum level of IgA were significantly associated with the development of AIDS. There was no association between disease progression and persistent generalized lymphadenopathy. When adjusted to the probable year of infection, these results are in accordance with previous cohort studies. It is concluded that most, or all, subjects seropositive for HIV will develop progressive loss of CD4 lymphocytes followed by clinical signs of immune deficiency, and that differences among previous cohorts with respect to disease progression are probably due to differences in the duration of infection.