A multicentre clinical trial was conducted in 114 Italian endoscopy centres in order to evaluate the comparative efficacy of four different ranitidine dosage regimens in the short-term treatment of active duodenal ulcer. Results were analysed in a total of 1745 patients randomly allocated to treatment with ranitidine 150 mg twice daily - morning and 19h30 (440 patients), ranitidine 150 mg twice daily - morning and 22h30 (438 patients), ranitidine 300 mg once daily at 19h30 (434 patients) or ranitidine 300 mg once daily at 22h30 (433 patients). The four groups were well matched for patient characteristics at entry. Initial treatment was for three weeks, with continuation to six weeks in cases with endoscopically unhealed ulcers at three weeks. Efficacy was evaluated in terms of endoscopic ulcer healing and control of pain symptoms in 24 h (daytime, nocturnal, daytime plus nocturnal). No statistically significant differences were found between any of the groups either as regards control of pain symptoms or ulcer healing rates (mean healing rates at three and six weeks were 77% and 98%, respectively). The results in this very large patient sample confirm equivalent efficacy of twice daily and single bedtime dose regimens and provide no evidence for superior efficacy of early evening compared with bedtime administration. In the population as a whole, concomitance of the three main risk factors (more than 20 cigarettes/day, ulcer size greater than 1 cm, deformation of the duodenal cap) was associated with a distinctly lower three-week healing rate (40.9% versus 87.4% in patients presenting none of these factors), though this difference tended to disappear at six weeks.(ABSTRACT TRUNCATED AT 250 WORDS)