A double blind trial was conducted in seven centres to evaluate the safety and efficacy of cimetidine 800 mg given at night compared with 400 mg given at breakfast and at bedtime. Altogether 197 patients with active duodenal ulcer confirmed by endoscopy entered the study, of whom 187 were eligible for analysis. After four weeks' treatment the ulcer was healed in 76 of 91 patients (84%) receiving the once daily regimen and in 65 of the 96 patients (68%) receiving the twice daily regimen (p less than 0.05). Both dosage regimens were equally effective in reducing ulcer pain and consumption of antacids. Pain relief was considerable within the first two weeks, and most of the patients were free of symptoms by the end of treatment. No patients were withdrawn because of adverse events as these were few and mild, consistent with the proved safety profile of cimetidine. Cimetidine 800 mg given at night is as effective as 400 mg twice daily; the single dose regimen may improve patient compliance, thus facilitating treatment.