For the evaluation of clinical efficacy of famotidine, a new, potent, long-acting histamine H2-receptor antagonist, in patients with gastric and duodenal ulcer, 42patients with endoscopically-proved active gastric ulcer (19 patients) and duodenal ulcer (23 patients) were admitted to this trial. Thirty two patients were treated with famotidine 20 mg b.i.d. and 10 with cimetidine 200 mg q.i.d. up to 8 weeks. The observed results were as follows: 1) The complete healing rates of gastric ulcer by endoscopic examination at 4,6 and 8 weeks were 50.0%, 68.8% and 93.8% in famotidine group, and 33.3%, 66.7% and 100.0% in cimetidine group, respectively. 2) The complete healing rates of duodenal ulcer by endoscopic examination at 4, 6 and 8 weeks were 62.5%, 81.3% and 100.0% in famotidine group, and 57.1%, 71.4% and 85.7% in cimetidine group, respectively. 3) Abnormal laboratory finding after treatment was found only in one case in famotidine group, but 3 cases in cimetidine group. 4) There was no significant difference in symptomatic improvement between two groups and mild side effects were complained of in 5 of 32 famotidine group (15.6%) and 4 of 10 cimetidine group (40. 0%). In conclusion, data from the present ivestigation suggest that Famotidine is effective in the treatment of gastric and duodenal ulcer, and is well tolerated on a short-term basis. Further and more extensive studies are necessary to confirm these findings.