In 39 patients with endoscopically healed duodenal ulcers repeat endoscopy and two antral biopsies after 1 year showed a relapse rate of 59%. Only post-treatment Campylobacter pylori status was a significant predictor of endoscopic relapse. 79% of patients who remained culture positive had a relapse, compared with 27% of culture-negative patients. Relapse was more likely (66%) in patients with a recurrence of C pylori after apparent eradication of the organism than in those who remained negative for C pylori (10%). No patient who remained negative for C pylori had histological gastritis, whereas all with recurrence of C pylori showed histological gastritis. These findings suggest an important role for C pylori in duodenal ulcer relapse in the year after treatment.