Famotidine was compared to ranitidine in a short-term study on the treatment of duodenal ulcer. Famotidine 20 mg. b.i.d., 40 mg. b.i.d. and 40 mg. nocte heal as many ulcer as ranitidine (90.9%, 91.7%, 83.3% and 100% respectively). A single 20 mg. bedtime dose shows to be effective on preventing ulcer recurrence for as long as 48 weeks; the 38% recurrence rate observed with famotidine was statistically different from the 78% observed with placebo. Diarrhoea was the most common complain observed during the short-term trial, followed by sleepiness and headache. The few and small biochemical alterations during the long-term treatment (increase in transaminases, alkaline phosphatase, glucose, BUN) could in no instance be directly related to the substances on use.