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