The aim of the present study was to investigate the effectiveness of ranitidine in the treatment of duodenal ulcer. Fourty patients with endoscopically proven pyloric or duodenal ulcer were treated with ranitidine 40 mg t.d. with meals and 80 mg nocte, or identical placebo tablets under double-blind conditions. Endoscopy after four weeks of treatment revealed complete healing in 15 out of 18 (83.3%) ranitidine-treated patients and in 5 out of 17 (29.4%) of the placebo patients (P less than 0.01). Ulcer symptoms were significantly less in ranitidine-treated patients, while the difference in antacid consumption between the two groups was found to be only arithmetical. No side effects or significant hematological or biochemical abnormalities were found. Four-week treatment with 200 mg of ranitidine daily seems to correspond to that of 6-8 weeks with 1-1, 6 g of cimetidine.