The objective was to study the impact of exposure group on the progression rate to the acquired immunodeficiency syndrome (AIDS). 289 subjects in Oslo, Norway, infected with the human immunodeficiency syndrome (HIV) and without major clinical signs of HIV infection (102 intravenous drug users, 151 homosexual men and 36 heterosexually infected subjects) were recruited to the Oslo HIV Cohort Study from 1989 and followed until 1 January 1995. 15 (14.7%) of the intravenous drug users, 56 (37.1%) of the homosexual men and 5 (12.5%) of the heterosexually infected subjects developed AIDS during a mean time of 47 months (p < 0.001, log rank test). When controlling for possible confounding variables (age, number of CD4+ lymphocytes, antiviral therapy at study entry, gender and year of HIV diagnosis), the relative risk of AIDS progression was 2.2 [1.1-4.5, 95% confidence interval (CI)] for homosexual men and 0.5 (0.2-1.3, 95% CI) for heterosexually infected subjects as compared to intravenous drug users. In a subgroup with known time of seroconversion (n = 60), 47% (18/38) of the homosexual men, 20% (3/15) of the intravenous users and none (0/7) of the heterosexually infected subjects developed AIDS (p = 0.04, log rank test). The results suggest that homosexual men have more rapid progression to AIDS than intravenous drug users and heterosexually infected subjects.