The incidence of Kaposi's sarcoma (KS) was examined with the use of data from the Surveillance, Epidemiology, and End Results (SEER) Program of the National Cancer Institute. KS is a manifestation of the recent epidemic of acquired immunodeficiency syndrome (AIDS) that has occurred particularly among homosexual men. The incidence of KS in 1973-79 was found to be higher (0.29 male and 0.07 female cases/100,000/yr) than is usually cited for the pre-AIDS KS incidence rates. Collectively, the 9 SEER registries in the United States showed only a slight increase in the incidence of KS between 1973-79 and 1980-81. However, the SEER registry covering San Francisco, which is a high-risk area for AIDS, showed a marked excess of KS in 1981. The KS case rate among never-married men younger than 50 years old, a surrogate index of homosexuality, was found to be markedly elevated in the post-AIDS period, compared with the case rate of a reference disease, mycosis fungoides. Never-married men younger than 50 years old, therefore, constitute a SEER-identifiable population who can be monitored for risk of KS and other neoplasms that might be related to AIDS. In addition, the incidence rate of KS in the SEER registry of Puerto Rico was generally higher than that in the U.S. SEER registries, despite data that suggested that KS may be underreported. The demographic characteristics of patients diagnosed as having KS in Puerto Rico suggested the classical rather than the AIDS-related form of KS.