OBJECTIVE: To assess the prevalence of gastric and duodenal bacterial colonization in HIV-infected patients. METHODS: Twenty-three consecutive outpatients at various stages of HIV infection were examined. No patient received antibiotic therapy or antisecretory drugs, and none presented with digestive symptoms. A second group consisted of 39 patients without risk factors for HIV infection referred to the gastroenterology outpatient clinic for suspected peptic ulcer disease. Gastric and duodenal juices were aspirated separately through the endoscope under direct visual control, using a sterilized double-sheathed tube. Specimens were plated quantitatively for both aerobic and anaerobic organisms. Parasitological evaluation was done on duodenal samples. RESULTS: gastric and/or duodenal bacterial colonization was documented in 7/23 (30%) HIV+patients and in 3/39 (8%) patients in the second group (p < 0.05). No parasites were detected. All isolates were oral Gram-positive cocci or bacilli. Mean fasting gastric pH was significantly higher in HIV-infected patients with bacterial colonization than in HIV-infected patients and patients in the second group without bacterial colonization (p < 0.02). There was no correlation between the presence of bacterial colonization and CD4+ cells counts. CONCLUSION: HIV infection may predispose to asymptomatic digestive bacterial colonization. Further studies are needed to assess the role of bacterial colonization in symptomatic patients presenting with diarrhea and/or weight loss.