Thirteen cases of disseminated Mycobacterium avium-intracellulare (MAI), representing a total of 27 bone marrow specimens, were studied. The patients ranged from 25 to 46 years of age and included ten immunocompromised and three immunocompetent hosts. Peripheral blood findings included anemia in all patients, leukopenia in 73%, thrombocytopenia in 45%, and pancytopenia in 45%. Fourteen of the specimens (52%) showed granulomas ranging from small, subtle lymphohistiocytic aggregates to larger lymphohistiocytic lesions and clusters of epithelioid histiocytes; almost half of these lesions were missed initially. Rare acid-fast bacilli were seen in only one case, but 53% grew MAI on culture. In one case of the acquired immunodeficiency syndrome, culture was positive in the absence of inflammation or AFB on staining. These findings are not significantly different from those reported in disseminated Mycobacterium tuberculosis infection.