Gastrointestinal function and structure in HIV-positive patients. We examined 19 patients (17 men) with human immunodeficiency virus (HIV) infection and gastrointestinal symptoms to determine whether those symptoms were due to either a gastrointestinal tract infection or a defect in mucosal absorption because of an enteropathy. The erythrocyte folate and serum vitamin B12 levels were within normal limits in all of the patients. The serum ferritin level was elevated in 12. The xylose absorption test results were abnormal in 8 of the 13 patients able to complete the study. None of the duodenal aspirates yielded a pathogen. Light microscopy revealed nonspecific lymphocytic inflammation without infection in the stomach (in seven patients), the esophagus (in five), the duodenum (in two) and the rectum (in two). However, biopsy specimens were positive for Candida albicans in the esophagus (four patients), cytomegalovirus in the esophagus (one) and the rectum (two), Helicobacter pylori in the antrum (two), Treponema infection in the rectum (two) and Mycobacterium avium-intracellulare in the small intestine (one). Only three patients had a normal series of biopsy specimens. All of the patients had similar ultrastructural changes at the epithelial-stromal junction of the antral glands and in the intestinal crypts. We conclude that abnormal biochemical and endoscopic findings are common in HIV-positive patients with gastrointestinal symptoms. Defects in carbohydrate absorption and ultrastructural changes may be responsible for some aspects of HIV enteropathy. 