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