A review of the discharge diagnoses and mycobacterial cultures of patients admitted to a major New York City hospital over an 18-month period revealed 21 patients with abdominal mycobacterial infections (17 male, 4 female) with an average age of 36 years. Acquired immunodeficiency syndrome (AIDS) or an identifiable AIDS risk was present in 14. The disease was manifest by peritoneal (eight patients), ileocecal (seven), and hepatic involvement (three), and psoas abscess (three). Diffuse abdominal pain was the most frequent presenting symptom. However, absence of pain (19 percent) and lack of abdominal findings (28 percent) were not uncommon. The erythrocyte sedimentation rate was significantly elevated (mean 72 mm/hour), whereas the white blood cell count was normal in 18 patients. Computed tomography findings were abnormal in all patients studied and suggested mycobacterial infection in 67 percent. Ten patients (48 percent) required surgery. Although there were no individual differences in clinical or laboratory presentation between the operative and nonoperative patient groups, more patients with pain and higher fever were operated upon. There was one postoperative death. The overall mortality rate was 24 percent, and the mean survival and follow-up 10.2 months and 12.2 months, respectively.