The authors conducted an analysis of all 677 cases of Kaposi's sarcoma among the 3,047 cases of acquired immunodeficiency syndrome diagnosed in homosexual/bisexual men in Canada between 1980 and 1989. The proportion with Kaposi's sarcoma declined from 32.2% during 1980-1985 to 15.0% in 1989. The proportion with Kaposi's sarcoma was significantly higher in primary epidemic centers (Vancouver, Toronto, and Montreal) and in men in the 1945-1954 birth cohort independent of year of diagnosis. These data are consistent with an environmental cofactor for Kaposi's sarcoma which is likely to be a sexually transmitted agent.