In order to study differences in progression to Acquired Immunodeficiency Syndrome (AIDS) between risk groups, 205 homosexual men and 185 intravenous drug users (IVDUs) were followed from diagnosed seropositivity for Human Immunodeficiency Virus Type-1 (HIV) for a mean period of 46 months (range 1-88 months). Seven (4%) IVDUs and 55 homosexual men (27%) were diagnosed with AIDS during the follow-up period. The probability of being AIDS-free four years after diagnosed HIV positivity was 0.96 for IVDUs (SE 0.02) and 0.73 (SE 0.04) for homosexual men (p < 0.001, log rank test). When controlling for age and gender, the relative risk of AIDS progression for homosexual men was 9.1 (3.5-24.1, 95% confidence interval) as compared with IVDUs. Even when 24 months of follow-up time without progression were added for all homosexual men, assuming that the epidemic started two years earlier in this group, the relative risk of progression was 5.4 (2.1-14.4, 95% confidence interval) for homosexual men.