The changing epidemiology of Kaposi's sarcoma (KS) and possible explanations for this change were analyzed using data from a well-characterized cohort of homosexual and bisexual men. Among 1,341 men with AIDS, the proportion presenting with KS declined from 79% in 1981 to 25% in 1989. For 250 men whose date of HIV seroconversion could be well characterized, persons presenting with KS had a shorter interval from HIV seroconversion to AIDS diagnosis than other AIDS patients without KS (mean = 77 vs. 86 months). Among 182 men who were interviewed prior to a diagnosis of AIDS, men with and without KS did not significantly differ with respect to number of sex partners, a history of certain sexually transmitted or enteric diseases, use of certain recreational drugs (including nitrite inhalants), or participation in certain specific sexual practices. The decline in KS may at least partly be due to a shorter latency period from infection to disease. Although cofactors for the development of KS may exist, many previously hypothesized agents were not supported by this analysis.