The aim of this study is to propose a regimen of omeprazole 20 mg twice weekly for the prevention of duodenal ulcer (DU) relapse. 257 endoscopically verified DU patients entered this study. They were divided into four groups: (1) Group A, 68 patients stopped treatment after healing of DU during treatment of cimetidine 800 mg at night. (2) Group B, 72 patients healed DU after treatment of omeprazole 20 daily. Then anti-ulcer drug was withdrawn. (3) Group C, after healing of DU, 54 patients received a maintenance dosage of cimetidine 400 at night for up to 1 year. (4) Group D, after healing of DU, 63 patients received omeprazole 20 mg twice weekly (every Monday and Thursday) for up to 1 year. All the patients were followed in out-patients department for assessment of symptomatic response and side effect. Endoscopic examination were performed in those patients when any symptom related to the upper gastrointestinal tract appeared. The cumulative recurrence rates of Group A to Group D at 6 months were 44.1%, 23.6%, 22.2% and 4.8%, and that at 12 months were 76.5%, 36.1%, 27.8% and 6.3% respectively. The relapse rate of patients with omeprazole 20 mg twice a week was significantly lower than that of other groups at either 6 or 12 months. In patients with maintenance therapy of cimetidine, DU recurred more rapidly among smokers than nonsmokers, but there is no significant difference between smokers and nonsmokers receiving maintenance therapy of omeprazole.(ABSTRACT TRUNCATED AT 250 WORDS)