Chronic Helicobacter pylori infection has been identified as a major risk factor for the subsequent development of gastric carcinoma On the basis of seroepidemiological studies the relative risk for infected persons was estimated to range between 3 and 6. Our study attempted to determine the relative risk of gastric carcinoma in H. pylori-infected individuals based on the histological evaluation of gastritis in gastric carcinoma patients in the light of a declining prevalence of H. pylori infection in Western countries. We histologically determined the H. pylori infection rate in 215 patients with early gastric carcinoma (tumor stage pT1), and compared it with that of 215 asymptomatic persons matched by age and sex who were tested by the 13C urea breath test. On the basis of these data an odds ratio of 16.7 (CI 9.6-29.1) was calculated for the relative risk of developing gastric carcinoma in H. pylori-infected people. The histological diagnosis of gastritis permits a separate risk assessment for patients with autoimmune gastritis, and by excluding these patients from the analysis we calculated an odds ratio for H. pylori-infected persons of 150 (CI 36.4-622.9). The endoscopic-histological diagnosis of H. pylori infection is associated with an increased risk of the subsequent development of gastric carcinoma of approximately 150-fold compared with H. pylori-negative patients who do not have chronic atrophic corpus gastritis of the autoimmune type (type A gastritis).