The presence of Helicobacter pylori (HP) in gastric biopsy specimens of 500 patients referred for routine upper gastrointestinal endoscopy for various abdominal complaints was investigated histologically and microbiologically. HP was detected in 429 of the 500 patients (86%). Antral biopsy specimens revealed gastritis in 457 out of 500 cases (91.4%). In the 43 patients who had normal histological findings, only 3 had HP infection (7%). The prevalence of HP in the patients with gastric and duodenal ulcers was 91%. In 95.6% of the ulcer patients, biopsy specimens showed gastritis. There was a statistically significant rise in the prevalence of HP with age. The correlation between histologic and microbiologic diagnostic methods was good. This study shows that HP positivity and gastritis are common in a routine endoscopy population and that there is a strong association between H. pylori, gastritis and peptic ulcer disease.