A multicenter study was carried out on a total of 574 patients to investigate the safety and efficacy of a 800-mg nighttime dose of cimetidine in comparison with 400 mg twice daily for 4 or 8 weeks in the treatment of patients with duodenal ulcer. No statistically significant differences were observed between the two regimens. Healing rates were as follows: at 4 weeks 212 of 269 patients (79%) healed with the once daily regimen whereas 200 of 270 patients (74%) healed with the twice daily dosage; at 8 weeks 242 of 251 patients (96%) healed with the once daily regimen as compared with 232 of 246 (94%) treated with the twice daily dosage. The dosages were equally effective in relief of pain during the study period. The results confirm that a single nighttime dose of cimetidine is as effective as the twice daily regimen.