The clinical setting, gross organ distribution, and microscopic pathologic findings of disseminated Mycobacterium avium-intracellulare (MAI) infection are described at autopsy in 12 patients with acquired immunodeficiency syndrome (AIDS). All patients were diagnosed by premortem mycobacterial cultures. The clinical course of MAI infection was often prolonged, and death was usually due to an additional infection. In every patient, the distinctive microscopic feature on hematoxylin--eosin staining was a poorly defined granuloma consisting of pale blue, striated histiocytes filled with mycobacteria. Well-formed granulomas with fibrosis, necrosis, and epithelioid histiocytes were present in less than one third of cases. MAI is an opportunistic pathogen that may complicate the course of AIDS but only rarely leads to death. The characteristic appearance of striated histiocytes may aid in the recognition of this infection.