A multicentre double-blind study was carried out on a total of 197 patients, to evaluate the safety and efficacy of an 800 mg nighttime dose of cimetidine in comparison with 400 mg twice daily in the treatment of duodenal ulcer. At 4 weeks 84% of the 187 patients eligible for analysis had healed ulcers with the once daily regimen and 68% with the twice daily regimen (p less than 0.05). An early decrease in both day and nighttime pain and in antacid consumption was observed during the first 2 weeks. Adverse effects were few and mild, confirming the safety profile of cimetidine.