Abstract: Mastocytosis encompasses a range of disorders characterized by overproliferation and accumulation of tissue mast cells. Mast cell disease is most commonly seen in the skin, but the skeleton, gastrointestinal tract, bone marrow, and central nervous system may also be involved. We present a 10-year-old boy with diffuse cutaneous mastocytosis characterized by disseminated papular, nodular, and infiltrated leathery lesions. The patient presented with chronic diarrhea and malnutrition. Laboratory studies were normal except for an elevated urinary 1-methylhistamine level. The bone marrow aspirate showed a dense mast cell infiltrate confirming systemic involvement.