A controlled study was conducted on patients with human immunodeficiency virus (HIV) infection referred for upper endoscopy to evaluate the prevalence of Helicobacter pylori (H. pylori) infection. Four different stains and culture for H. pylori were performed on biopsy specimens from the gastric antrum. Sixteen (40%) of 40 patients with acquired immune deficiency syndrome (AIDS) or AIDS-related complex (ARC) were diagnosed to be infected with H. pylori versus 14 (39%) of 36 age-matched control patients. Eight of 15 AIDS/ARC patients without AIDS-related esophagogastroduodenal findings (53%) were infected with H. pylori versus 8/25 (32%) with endoscopic findings typical of AIDS. No invasion of the lamina propria by H. pylori was noted in any patient. Active chronic gastritis was present in 60% of AIDS/ARC patients and 61% of controls. Fifty-eight and 59%, respectively, of active chronic gastritis cases were infected with H. pylori. All the H. pylori infections, except one, were found in patients with chronic gastritis. In AIDS/ARC patients, H. pylori infection and active chronic gastritis are as common as in other patients referred for upper endoscopy. They may play a pathogenic role, especially when endoscopic AIDS-related findings are lacking. Cell-mediated immune deficiency does not appear to increase the risk of infection with H. pylori.