Abstract: Mastocytosis is a broad term used for a group of disorders characterized by accumulation of mast cells in the skin with or without extracutaneous involvement. The clinical spectrum of the disease varies from only cutaneous lesions to highly aggressive systemic involvement such as mast cell leukemia. Mastocytosis can present from birth to adulthood. In children, mastocytosis is usually benign, and there is a good chance of spontaneous regression at puberty, unlike adult-onset disease, which is generally systemic and more severe. Moreover, individuals with systemic mastocytosis may be at risk of developing hematologic malignancies. We describe a girl who presented to us with a solitary mastocytoma at age 5 and later developed maculopapular cutaneous mastocytosis. At age 23, after an episode of anaphylactic shock, a bone marrow examination revealed mast cell leukemia. She ultimately died despite aggressive chemotherapy and bone marrow transplantation.