BACKGROUND: The incidence of Kaposi's sarcoma (KS) and non-Hodgkin's lymphoma (NHL) in the general population has markedly increased since the onset of the AIDS epidemic in 1981. However, during the 1990s, the dynamics of the AIDS epidemic changed, as human immunodeficiency virus (HIV) infection rates slowed and effective antiretroviral therapies were introduced. We examined the impact of these changes on the general population incidence of KS and NHL. METHODS: Age-standardized incidences for KS and NHL from 1973 through 1998 were obtained from nine population-based cancer registries that participate in the Surveillance, Epidemiology and End Results (SEER) program. RESULTS: During the mid-1990s, KS incidence declined sharply in all nine registries. Decreases in KS incidence were most evident in San Francisco, where KS rates among white men had risen from 0.5 per 100 000 people per year in 1973 to between 31.1 and 33.3 from 1987 through 1991 and then declined to 2.8 in 1998. With background NHL incidence in the general population being much higher than that for KS, changes in incidence related to the AIDS epidemic were most evident in subgroups at high risk of AIDS. In San Francisco, NHL rates among white men rose from 10.7 in 1973 to a peak of 31.4 in 1995 and then declined to 21.6 in 1998. NHL types that were most highly AIDS-associated declined most steeply, whereas the incidence of NHL types not associated with AIDS was either stable or increasing. CONCLUSION: Changes in KS and NHL incidence since the mid 1990s may reflect declines in the number of individuals with AIDS and improved immune function in such individuals following the introduction of effective antiretroviral therapies in the 1990s. Notably, non-AIDS-associated NHL incidence has continued to increase steadily through 1998.