A causative role is now accepted for Helicobacter (formerly Campylobacter) pylori in type B gastritis, and evidence is accumulating that H. pylori infection plays a major contributory role in duodenal ulcer, gastric ulcer, and epidemic gastric cancer. The prevalence of H. pylori in any population remains unknown. We compared the prevalence of H. pylori infection in the Riyadh region of Saudia Arabia, using a specific and sensitive ELISA for IgG antibody against the high molecular weight cell-associated antigen of H. pylori (urease). Subjects were interviewed, demographic data were collected, and a serum sample was obtained. Subjects completed a questionnaire that included questions about level of education, smoking, medications used, presence and frequency of symptoms referable to the upper gastrointestinal tract, and family history of ulcer disease. We studied 557 individuals (ranging in age from 5 to 91 yr). The prevalence of H. pylori infection increased rapidly with age: from 40% of those ages 5-10 yr, to more than 70% of those 20 or older. H. pylori infection occurred with significantly more frequency in adults with less than 12 yr of schooling, compared with adults who had attended college. The high rate of acquisition of H. pylori infection in Saudia Arabia emphasizes that studies of H. pylori-disease associations must consider the baseline prevalence of H. pylori infection in that population.