Two hundred and fifty patients were included in a double-blind, parallel, randomized, controlled clinical trial. Duodenal ulcer treatment lasted up to 8 weeks. Forty-nine patients were followed up for prevention of ulcer relapse for up to one year. All patients received either ranitidine (300 mg/day in the healing phase and 150 mg/day in the follow-up phase) or ebrotidine (N-[(E)-[[2-[[[2-[(diaminomethylene)amino]-4 -thiazolyl]methyl]thio]ethyl]amino]methylene]-4-bromo-benzenesulfonamide , CAS 100981-43-9, FI-3542) (400 mg/day in both phases) as a single dose at bedtime. Both groups were matched in all demographic parameters, except for a significantly higher percentage of smokers in the ranitidine group. The percentage of total healing was almost the same with both products. Healing occurred in a higher percentage with ebrotidine at weeks 4 (75% versus 66.7%) and 6 (87% versus 79.7%). A higher effect of ebrotidine on the incidence of duodenitis was identified during the whole study, but only reached statistical significance at week 6. The relapse rate during the follow-up phase showed no differences between the two study treatments, relapse percentage figures being 25% for ebrotidine and 24% for ranitidine. There were no differences in the number of unscheduled visits between the two groups, although 57% of patients in the ranitidine group had to make a second follow-up visit, as compared with 33% in the ebrotidine group. Both drugs caused hardly any side effects, affecting only one patient from each group: one patient with ebrotidine suffered from diarrhoea and one patient with ranitidine developed a skin rash on the limbs. Administration of ebrotidine in a single dose (400 mg/d) was at least as effective and safe as ranitidine both for healing and relapse prevention in patients with duodenal ulcer.