Patients with acquired immunodeficiency syndrome (AIDS) frequently have diarrhea and weight loss. We prospectively examined the upper and lower gastrointestinal tracts in 22 AIDS patients, although severe medical problems often precluded full evaluation. Ninety-six percent (21 of 22) lost weight, and 55% (12 of 22) had diarrhea. The mean (+/- SD) weight loss was 34 +/- 19 lb. Steatorrhea was found in 4 of 14 patients, and D-xylose tests were abnormal in 8 of 14 patients. Mean serum albumin was 3.3 +/- 0.8 g/dl. A significantly diminished plasma selenium level, which can influence immune function, was noted in these AIDS patients. Gastrointestinal infections were identified in 45% of patients. Although diarrhea and malabsorption were more common in the infected group, weight loss and albumin were similar in those with and without demonstrated infections. Flexible sigmoidoscopy showed that of 15 patients, there were two with Kaposi's sarcoma, 10 normals, and three with nonspecific endoscopic changes of colitis. Infection was documented in all patients with colitis. Panendoscopy of the upper gastrointestinal tract was positive for AIDS-related pathology in five of 10 patients, including two with Kaposi's sarcoma, one with Candida esophagitis, one with herpetic esophagitis, and one with gastroduodenitis (biopsy positive for cryptosporidia); five patients had a normal-appearing tract. Small bowel or colonic biopsies frequently showed nonspecific inflammatory changes, although pathogens were identified in six patients (27% of all biopsies). We conclude that a wide variety of gastrointestinal pathology, which includes infectious agents, neoplasms, and inflammatory changes, may occur in AIDS patients. Therefore, AIDS patients, particularly those with diarrhea or weight loss, deserve an intensive evaluation for remediable lesions of their gastrointestinal tracts.