Patients with symptoms of gastritis or peptic ulcer disease were recruited to study the prevalence of Campylobacter pylori. On the basis of the endoscopic diagnosis only, the isolation rates of the organisms in normal, gastritis or gastroduodenitis (GD), and peptic ulcer (PU) disease patients, were not significantly different among the 89 patients evaluated. However, analysis based on histopathological evaluation (in 73 out of these 89 patients) revealed a significant difference (p = 0.00016) in the isolation rate between histologically normal individuals (14%) and those with GD (89%). Also, a significant (p = 0.03) difference was observed in the C. pylori detection rate among patients with GD and those with PU disease (61%). After adjustment for multiple comparisons, only the difference in C. pylori prevalence between normal and GD patients was maintained. After conventional therapy, 23 patients who initially had GD or PU disease were submitted to re-endoscopy. In the latter group, a correlation between presence or absence of organisms and histological healing was noted. The potential pathogenesis of C. pylori in gastritis and peptic ulcer disease, and designs for future trials are discussed.