Forty patients with chronic duodenal ulcer who had healed endoscopically with a 6-week course of cimetidine were randomized double blind to 1 year of either placebo or cimetidine tablets 400 mg bid (20 patients in each group). Patients were seen at monthly intervals, and endoscopy was performed at clinical relapse or on completion of 1 year. One of 20 patients on active cimetidine relapsed clinically and endoscopically at 3 months; 16 of 20 patients on placebo relapsed clinically and endoscopically within 9 months, the majority within 3 months, and 2 were shown to have asymptomatic chronic ulcers at routine 12-month endoscopy. None of the 19 patients on active cimetidine routinely endoscoped at 12 months showed evidence of ulceration. This study confirms a high relapse rate when short-term cimetidine is ceased and indicates that maintenance treatment with cimetidine prevents relapse.