The objective of this study is to describe participants in the Multicenter AIDS Cohort Study (MACS) with incident infection due to the human immunodeficiency virus type-1 (HIV-1) in whom AIDS developed by March 1990 and within 5 years of seroconversion (group A). Secondly, behavioral, clinical, and immunologic characteristics of these men are compared to those of matched seroconverters remaining AIDS free (group B). Between entry into the MACS (April 1984-March 1985) and July 1989, 345 seronegative homosexual/bisexual men had HIV-1 antibody; of these men, AIDS developed in 32 by March 1990. The Kaplan-Meier estimates of the proportion of men with incident HIV-1 infection with AIDS were 6 months, 0%; 12 months, 1%; 24 months, 3%; and 48 months, 10%. These 32 men engaged in receptive anal intercourse with more partners before (p< 0.005) and after seroconversion (p< 0.005) and reported more sexually transmitted disease preseroconversion (p= 0.05) than did group B. These findings suggest that sexually transmitted co-factors, preseroconversion and/or postseroconversion, play a role in the progression of HIV-1 infection. Alternatively, greater sexual activity could increase the hazard of exposure to a more virulent HIV-1 strain.