The present study was undertaken to compare the efficacy of ranitidine 150 mg/nocte with that of antacids taken "on demand" in the prevention of recurrence of duodenal ulcer (DU). Two groups (A, B) of patients with DU recently healed by ranitidine or antacids were studied. Patients of Group A (110) healed by ranitidine were put on maintenance treatment with ranitide, and patients of Group B (32) healed by antacids were given antacids "on demand" for a period of one year. Endoscopy was performed at the end of the 6th and 12th months, as well as whenever symptoms appeared. Patients were followed up clinically at 2-month intervals. Relapse rates of 11% (group A) and 34% (group B) were found at the end of the 6th month, (p less than 0.005), while the total relapse rates at the end of the 12th month were 19% and 47%, respectively (p less than 0.005). Recurrences were asymptomatic in 62% (Group A) and 27% (Group B) of patients. No significant side effects were observed. It is concluded that ranitidine 150 mg nocte is significantly superior to antacids "on demand" as far as the recurrence of DU is concerned.