In the United States Kaposi's sarcoma is at least 20000 times more common in persons with acquired immunodeficiency syndrome (AIDS) than in the general population and 300 times more common than in other immunosuppressed groups. Among persons with the acquired immunodeficiency syndrome (AIDS) reported to Centers for Disease Control by March 31, 1989, 15% (13 616) had Kaposi's sarcoma. Kaposi's sarcoma was commoner among those who had acquired the human immunodeficiency virus (HIV) by sexual contact than parenterally, the percentage with Kaposi's sarcoma ranging from 1% in men with haemophilia to 21% in homosexual or bisexual men. Women were more likely to have Kaposi's sarcoma if their partners were bisexual men rather than intravenous drug users. Kaposi's sarcoma risk was not consistently related to age or race but varied across the United States, being greatest in the areas that were the initial foci of the AIDS epidemic. Thus Kaposi's sarcoma in persons with AIDS may be caused by an as yet unidentified infectious agent, transmitted mainly by sexual contact.