Mastocytosis is characterized by increased proliferation of mast cells. Two patients had systemic mastocytosis involving the skin and gastrointestinal tract, complicated by malabsorption and tetany. Absorption studies in these patients suggested that the entire small bowel was involved and that the defect was mild in the absence of diarrhea. Small bowel biopsies disclosed infiltration of the lamina propria and submucosa by mast cells, and gastrointestinal tract x-ray films showed nodular densities, edema, and thickening of the bowel wall. Tetany was due in part to combined hypocalcemia, hypomagnesemia, and hypokalemia. Diarrhea and malabsorption were due to mast cell infiltration of the bowel rather than to histamine. patients with signs of systemic mastocytosis should have careful evaluations and be followed up to prevent development of malabsorption and tetany.