PIP: This review of the epidemiology of acquired immunodeficiency syndrome (AIDS) focuses on the AIDS virus, immunology, clinical manifestations, surveillance, epidemiologic studies, and prevention. Although no treatment has been able to reverse the immunodeficiency characteristic of AIDS, epidemiologic studies have identified the routes of transmission of this virus and thus suggest strategies for prevention. The number of reported cases of AIDS in the US is increasing rapidly. The 1st 1000 cases were reported over 17 months, the next 2000 were accumulated over 12 months, and an additional 3000 were recorded in only 11 months. 5 groups of patients account for 93% of all AIDS cases: homosexual and bisexual men (73%), intravenous drug abusers (17%), hemophiliacs (1%), sexual contacts of these groups (1%), and transfusion recipients (1%). Opportunistic infections and cancers are not equally distributed among the different risk groups, leading to speculation that a cofactor such as cytomegalovirus infection or exposure to nitrate inhalants may predispose individuals with AIDS to develop Kaposi's sarcoma, for example. Epidemiology has contributed significantly to the understanding and control of AIDS. In addition, AIDS research has contributed to the field of epidemiology, pointing to the need for combined efforts of acute and chronic disease epidemiologists as well as coordination with other specialties. Studies of AIDS have also pointed to the need for further epidemiologic research on the health problems of homosexual men, intravenous drug abusere, hemophiliacs, and transfusion recipients.