We examined data from San Francisco and other areas participating in the Surveillance, Epidemiology, and End Results (SEER) Program to determine the effect of the human immunodeficiency virus (HIV) epidemic on cancer incidence between 1973 and 1987. In this period, non-Hodgkin's lymphoma incidence has increased over 10-fold and Kaposi's sarcoma incidence has increased over 5000-fold in single San Francisco men 20 to 49 years of age. Increases in non-Hodgkin's lymphoma have been restricted to high-grade and diffuse large-cell (intermediate-grade) histological types. With the exceptions of non-Hodgkin's lymphoma and Kaposi's sarcoma, no other tumor has significantly increased in incidence. During 1987, we estimate that HIV-seropositive men in San Francisco had a 0.47% risk of developing non-Hodgkin's lymphoma and a 1.6% risk of developing Kaposi's sarcoma. The relative risks for non-Hodgkin's lymphoma and Kaposi's sarcoma associated with HIV infection were 104 and 40,000, respectively. For 1987, HIV was associated with 14% of all reported cancers (except non-melanoma skin cancer) in men aged 20 to 49. We expect that 1,890 to 2,730 excess cases of non-Hodgkin's lymphoma and 6,490 to 8,320 excess cases of Kaposi's sarcoma will occur in the United States in 1990.