Lactase deficiency (LD) has been described in association with peptic ulcer disease of the upper gastrointestinal tract, but little has been known as to the incidence of LD in the patients with ulcer disease of the upper GI tract or with gastric carcinoma. The purpose of the present study is to investigate the incidence of LD in gastric and/or duodenal ulcer disease and in gastric carcinoma, and to hypothesize the possible effect of these diseases on Ld. Clinical materials consisted of 40 cases of active duodenal ulcer disease, 19 cases of benign gastric ulcer, 5 cases of multiple ulcers both in the stomach and duodenum, and 32 cases of gastric carcinoma. We used the lactose-barium test in diagnosing LD. X-ray findings were assessed according to the criteria described by Laws et al. and Preger and Amberg in the small-bowel film obtained at 30 minutes after the ingestion of some 200ml of lactose-barium meal which contained 50gm of lactose. Our clinical study revealed that the i cidence of LD in duodenal ulcer was 50%, in gastric ulcer 57.9%, in gastric and duodenal ulcer 60%, and in gastric carcinoma 46.9%. The difference of incidence between each disease were statistically not significant, but the disease group and normal control was highly significant. The incidence of LD in disease group was nearly twice as high as that of normal control(27.7%). It is speculated that high incidence of LD in peptic ulcer disease of upper GI tract in particular may be associated with high acidity, the analog of which is found in Zollinger-Ellison syndrome. But high scidity is not a prominent feature in gastric carcinoma which is attended by almost equally high incidence of LD. It is therefore likely that the intolerability to lactose is caused by much complicated mechanism of versatile factors.