Helicobacter pylori has been shown to infect the gastric mucous layer of almost all patients with duodenal ulcer disease, as well as that of most patients with gastric ulcer disease. Recent studies have suggested that the eradication of H. pylori affects the natural history of duodenal ulcer disease such that the rate of relapse decreases markedly. We evaluated the relationship between H. pylori infection and peptic ulcer relapse in a Japanese population following a prospective study. Seven of 18 (38.9%) gastric ulcer patients positive for H. pylori relapsed by the end of 1 year, whereas only 1 of 9 (11.1%) gastric ulcer patients without H. pylori developed ulcer relapse (P < 0.05). Relapse rates of duodenal ulcer patients negative for H. pylori were significantly lower than those positive for H. pylori within 1 year (0% vs 66.7%; P < 0.01). The effects of anti-H. pylori drugs on the eradication of H. pylori were examined in 50 patients with peptic ulcers. Eradication rates with a proton pump inhibitor (PPI) alone (omeprazole 20 mg) showed the lowest values (4 of 13; 30.8%). The rates were: 44.4% for amoxicillin alone (4 of 9); 70% for triple therapy consisting of amoxicillin, metronidazole, and bismuth subnitrate (14 of 20); and 87.5% for concomitant therapy of the PPI plus amoxicillin (7 of 8). Reinfection rates of H. pylori within 1 year after eradication of this organism were distinctly higher in the PPI alone group (80%) than in other groups (18.2%-32.4%).(ABSTRACT TRUNCATED AT 250 WORDS)