Necropsy findings in 101 adult patients with the acquired immunodeficiency syndrome (AIDS) from two metropolitan hospitals were compared retrospectively with the antemortem clinical diagnoses. 94% of the patients were male and 68% were homosexual or bisexual. 75 (74%) patients had AIDS-related diseases at necropsy that were not suspected clinically. The commonest of the unsuspected AIDS-related diseases were cytomegalovirus infection (49% of all cases), systemic fungal infection (20%), systemic Kaposi's sarcoma (14%), Mycobacterium avium intracellulare infection (11%), and systemic herpes infection (9%). Cryptococcal infection and cytomegalovirus retinitis were always diagnosed antemortem; and Pneumocystis carinii pneumonia went undiagnosed in only 5 of 58 (9%) patients who had proven infection either clinically or at necropsy. 8 patients who died with fungal pneumonia had undergone bronchoscopy; however, in only 1 patient was it diagnosed antemortem. Tuberculosis was undiagnosed in 4 patients. 4 cases of central nervous system lymphoma diagnosed only at necropsy had been treated empirically for toxoplasmosis. Bacterial pneumonias contributed considerably to mortality in 30% of the patients.