Recent studies have shown that a single dose of ranitidine given for 2 weeks at 6 p.m. resulted in a higher healing rate of duodenal ulcer than the same dose given at 10 p.m. Our study was designed to confirm these results in a large population in France. Three hundred and fifty patients with endoscopically proven duodenal ulcer were randomly assigned to open treatment with ranitidine 300 mg, immediately after dinner (dinner group), or at bedtime (bedtime group). Endoscopy was performed after 2 and 4 weeks. Forty six patients were excluded from analysis (default: 3, date of endoscopies not respected: 43). Of the 304 patients analysed (mean age: 45.5 years, sex ration M/F: 3.2), 146 received ranitidine after dinner, 158 at bedtime. Age, sex, ethnic groups, smoking habits and alcohol consumption were comparable in the two groups. At endoscopy, before treatment, the mean diameter of ulcers was greater in the bedtime group than in the dinner group (bedtime: 9.6 mm, dinner: 7.9 mm). Healing rates after 2 weeks were 58 p. 100 in the dinner group and 40 p. 100 in the bedtime group (p = 0.002). After 4 weeks treatment, cumulative healing rates were 87.5 p. 100 and 83.5 p. 100, respectively. Smoking had an influence on healing after 2 weeks but not after 4 weeks of treatment. In conclusion, a single dose of 300 mg of ranitidine resulted in a higher healing rate of duodenal ulcer when given immediately after dinner than when given at bedtime.