Abdominal CT scans of 71 patients with AIDS who had proved disseminated infection due to Mycobacterium tuberculosis (27 patients) or Mycobacterium avium-intracellulare (44 patients) were reviewed retrospectively to identify radiologic features that can be used to distinguish the two infections. CT findings in patients with disseminated M. tuberculosis included focal lesions in the liver (11%), spleen (30%), kidneys (19%), pancreas (7%), and gastrointestinal tract (15%) and lymph nodes with central or diffuse low attenuation (93%). CT findings in patients with disseminated M. avium-intracellulare included marked hepatomegaly (20%); marked splenomegaly (14%); focal lesions in the liver (9%), spleen (7%), and kidneys (2%); diffuse jejunal wall thickening (18%); lymph nodes with central low attenuation (14%); and enlarged lymph nodes exclusively of homogeneous soft-tissue density (55%). The presence of focal visceral lesions and low-attenuation lymph nodes suggests disseminated M. tuberculosis, whereas marked hepatic and splenic enlargement, diffuse jejunal wall thickening, and enlarged soft-tissue-density lymph nodes suggest disseminated M. avium-intracellulare. Recognition of these CT features can lead to a tentative diagnosis so that appropriate therapy can be instituted before the results of mycobacterial cultures become available.