Mastocytosis is a disease characterized by excessive numbers of mast cells in skin, bone marrow and internal organs such as liver, spleen and lymph nodes. Its genesis appears to be related to somatic mutations in c-kit, the receptor for mast cell growth factor. Although most patients present with the indolent form of the disease, approximately one-third of the patients have an associated hematologic disorder such as a myeloproliferative state or myelodysplastic syndrome. Patients with advanced forms of the disease, including those with an associated hematologic disorder have a poorer prognosis than those with indolent disease. There is no treatment known to cure or improve the natural course of mastocytosis. Since mast cells arise in the bone marrow from a CD34+ progenitor, bone marrow transplantation may offer the only hope for a cure.