Incidence rates of AIDS illnesses are described among patients with < or = 100 CD4 cells/mm3 grouped by use of antiretrovirals and chemoprophylaxis. Data were obtained from 2646 homosexual men infected with human immunodeficiency virus type 1. Participants were in the Multicenter AIDS Cohort Study during 1985-1993. The incidence rates per 100 person-years for Pneumocystis carinii pneumonia were 47.4 without treatment, 21.5 with antiretrovirals alone, and 12.8 with antiretrovirals combined with chemoprophylaxis. For Kaposi's sarcoma these rates were 23.2, 11.3, and 15.1, respectively. The incidence of some opportunistic infections, including Mycobacterium avium complex, nonretinitis cytomegalovirus disease, and cytomegalovirus retinitis, increased among persons receiving P. carinii pneumonia prophylaxis, because of reduction of this pneumonia and extension of life span. The incidence pattern of AIDS-defining illnesses in patients receiving treatment points to the changing AIDS epidemic and the need for new therapies. The data are particularly relevant to the development and planning of clinical trials and to health care providers.