This paper will describe a case series of 35 AIDS patients with infection of the gastrointestinal (GI) tract with Mycobacterium avium-intracellulare (MAIC). Thirty-five homosexual men with a mean age of 35 yr and a mean duration of AIDS of 7.7 months prior to the diagnosis of MAIC were investigated to determine the extent of MAIC infection. The investigations included upper endoscopy, sigmoidoscopy, liver biopsy, bone marrow aspiration and biopsy, stool and blood cultures for MAIC, and D-xylose absorption tests. Tissue biopsy material was examined by light microscopy with the Ziehl-Neelsen stain. The duodenum was most commonly involved (30/34 men), with 65% positive on special stains and 76% positive on culture of biopsy tissue. Unusual fine white nodules, believed to be characteristic for duodenal MAIC infection, were observed in 12 men. Esophageal (two men), liver (two men), and rectal involvement (seven men) were found. In nine of 18 men (50%), the D-xylose test was abnormal. In 28 of 33 men (85%), blood cultures grew MAIC. Similarly, in 25 of 28 men (89%), bone marrow biopsies grew MAIC, and in 18 of 21 men (86%) stool samples grew MAIC. We conclude that GI tract infection with MAIC in AIDS patients is frequently associated with systemic infection with the agent. Duodenal involvement is common, and may be accompanied by a characteristic gross lesion, that of fine white nodules on the mucosa. Malabsorption, as determined by the D-xylose test, is not a universal finding, as has been reported previously.