In order to study differences in risk of the development of AIDS in different groups of HIV infected subjects, 151 homosexual men, 110 intravenous drug users (IVDUs) and 36 heterosexually infected persons without major signs of HIV infection at entry to the study were enrolled in a cohort study. The mean follow-up time was 35 months. At the end of follow-up 40 subjects (13%) were diagnosed as having AIDS. This represented 20% (31/151) of the homosexual men, 7% (8/110) of the IVDUs and 3% (1/36) of the heterosexual subjects. The probability of being AIDS-free 36 months after entering the study was 0.88 (0.84-0.92, 95% CI) for the total study population, 0.83 (0.77-0.90) for the homosexual men, 0.92 (0.86-0.99) for IVDUs and 0.93 (0.91-1.0) for heterosexual subjects (p < 0.05, log rank test). In a Cox regression analysis, adjusting for CD4+ cell count at study entry, the relative risk of AIDS progression was 2.4 (1.1-5.2) for homosexual men and 0.3 (0.04-2.4) for heterosexual subjects, compared with IVDUs. The results demonstrate a higher risk of AIDS for homosexual men during the follow-up period.