Patients with recently healed duodenal ulcers were enrolled at 14 participating centers in a 12-month study comparing the effectiveness of ranitidine and cimetidine, two H2-receptor blockers, for maintenance therapy. Patients were randomly assigned to take bedtime doses of 150 mg of ranitidine (n=60) or 400 mg of cimetidine (n=66). Endoscopic examinations were scheduled at baseline and after 4, 8, and 12 months of therapy, or when symptoms compatible with active ulcer disease developed. Life-table analysis indicated a relapse rate of 16% for the ranitidine subjects and 43% for the cimetidine subjects during the 12-month period (p=0.01). Therapy was discontinued in one ranitidine subject and two cimetidine subjects for adverse events considered drug-related. There were no drug-related laboratory abnormalities in either treatment group. No significant drug-related adverse effects were seen with either drug during this 1-year trial. At the doses prescribed, ranitidine was superior to cimetidine as maintenance therapy in duodenal ulcer disease.