Diamox(Acetazolamide) has been known to inhibit acid secretion iq the gastric mucosa and to cause leakage back of acid already secreted through the mucosa. Recent reports showed that it was able to differentiate between benign ulcer and malignant ulcer by using of large doses of Diamox(Acetazolamide). A clinical observation was done on 45 cases of peptic ulcer and 3 cases of gastric cancer who were treated with large doses of Diamox(Acetazolamide) 25 30mg per kilogram of body weight daily at Chonnam University Hospital, Kwangju from July, 1976 through August, 1977, The results were summerized as follows: 1) Of the 48 cases, 42 cases (87. 5%) showed an improvement in symptomatic and subjective symptoms. 2) Of the 25 cases of gastric ulcer, 13 cases (52%) showed complete symptomatic improvement and 1I cases (44%)showed) symptomstic improvement, but 1 case (4%3 was not improved. 3) Of the 20 cases of duodenal ulcer, 8 cases (40%) showed complete symptomatic improvement and 10 cases (50%) showed symptomatic improvement, but 2 cases (10Po) were not improved. 4) Three cases of gastric cancer which were confirmed by punch biopsy showed no improvent. 5) Side effects of Diamox(Acetazolamide) treatment were epigastric fullness, general weakness, weight loss, dizziness and tingling sensation of extremities. According to the above mentioned results, Diamox(Acetazolamide) treatment in the peptic ulcer was effective and was very useful for differentiation between benign ulcer and malignant ulcer.