Fifty-eight patients with endoscopically verified duodenal ulcers were treated with cimetidine, 1 g/day, for 6--10 weeks. The ulcers healed in 52 out of the 54 patients completing the treatment (96.3%). Forty-seven of the patients with healed ulcer were randomly allocated to 1 year of maintenance treatment, 23 patients with cimetidine, 400 mg at night, and 24 patients with placebo tablets. In the cimetidine-treated group 2 out of the 20 patients (10%) completing the trial had recurrence of ulceration, whereas 16 out of the 23 patients (70%) completing the placebo treatment had ulcer recurrence (p less than 0.001). The drug was well tolerated and, except for a marked increase in serum transaminases in three patients, no serious side effects were seen.