Gastrointestinal examinations in 12 patients with systemic mastocytosis have revealed peptic ulcer in 5 instances and small intestinal lesions in 5 patients. The principal bowel abnormalities are small mucosal nodules and diffuse thickening of the bowel wall. Intestinal biopsies obtained in 3 of these patients were reviewed. Correlation of the roentgen and pathologic findings indicates that the bowel wall thickening is due to infiltration of the lamina propria by a cellular infiltrate containing, as a rule, increased numbers of tissue mast cells. Marked submucosal edema in 1 patient produced an unusual small bowel pattern. Biopsy specimens failed to indicate the nature of the nodular mucosal lesion. Laboratory evidence of malabsorption was found in 2 of the 5 patients, 1 of whom had significant hypocalcemia. Awareness of the intestinal lesions in mastocytosis permits more adequate evaluation of the gastrointestinal symptoms which are common in these patients, assists in earlier recognition of malabsorption when it occurs, and may aid in the original diagnosis of systemic mastocytosis in certain instances.