Twenty-five cases of acquired immune deficiency syndrome (AIDS) complicated by opportunistic infections were diagnosed at Lenox Hill Hospital during an 18 month period and followed for at least 1 year or until the patients' deaths. Twenty-three patients were homosexual men, including 1 i.v. drug user, and 2 were heterosexual i.v. drug users, including the sole woman. Seventy infections were diagnosed. The commonest etiologic agents included Candida albicans, Pneumocystis carinii, cytomegalovirus, and Mycobacterium avium-intracellulare. Less frequent pathogens included Cryptococcus neoformans, Toxoplasma gondii, Cryptosporidium, JC virus, Mycobacterium xenopi, and Mycobacterium tuberculosis. Seven men also had Kaposi's sarcoma. Prodromal symptoms lasted up to 8 months before the diagnosis of AIDS. Immunologic and serologic evaluation demonstrated lymphopenia, polyclonal hypergammaglobulinemia, cutaneous anergy, reversal of the T-cell helper/suppressor ratio, and serologic evidence of previous exposure to CMV, EBV, and HBV. Pneumocystis pneumonia was diagnosed during life in 17 patients, by transbronchial biopsy and bronchoscopic washings, and all 17 cases were successfully treated. However, pneumocystis was more refractory to treatment than previously described and required an average of 29 days of antimicrobial therapy. Disseminated infections with atypical mycobacteria and cytomegalovirus were the leading causes of death. Bone marrow aspirates and biopsies often revealed a cellular abnormality (82%) but only occasionally yielded an infectious diagnosis (32%). However, bone marrow examinations were the major means of detecting atypical mycobacteriosis. Colonoscopic biopsies were most useful for establishing the presence of cytomegalovirus colitis. Transbronchial biopsies and bronchial washings gave a high yield of opportunistic pathogens including 34 infectious diagnoses confirmed by 31 procedures. Multiple site biopsies and close communication between the clinician and the pathologist were needed for early diagnosis of opportunistic infections. Twenty-two patients (88%) died, and the few survivors remain debilitated.