Systemic mastocytosis is characterized by mast cell proliferation in skin, bones, lymph nodes, and parenchymal organs. Patients usually present with classic dermatologic findings of urticaria pigmentosa. Typical symptoms include pruritus, flushing, tachycardia, asthma, and headache, all thought to result from the release of histamine from the mast cells (612). Fifty to eighty percent of patients have GI symptoms, including peptic ulcers, malabsorption, steatorrhea, nausea, vomiting, copious watery diarrhea, and abdominal pain (613). The clinical features may mimic inflammatory bowel disease. These features occur secondary to gastric hypersecretion or result from the release of histamine and prostaglandins from mast cells. Symptoms are often induced by alcohol consumption. The hyperhistaminemia produces gastric hypersecretion, which can be as marked as that seen in Zollinger-Ellison syndrome (613). Gastric acid levels correlate with the degree of histaminemia and with the presence of acid peptic disease, including peptic duodenitis (613). Mast cell–mediated events include increased intestinal permeability and altered smooth muscle function. Malabsorption occurs secondary to mucosal infiltration.