This protocol is designed to examine those growth potentiating and inhibiting factors which regulate mast cell number and survival in patients with mastocytosis, and to explore the molecular basis of the disease process in hopes of improving therapy. Patients will carry the diagnosis of mastocytosis based on abnormal bone marrow biopsy and aspirate, abnormal skin biopsy, presence of urticaria pigmentosa, and if available, elevated serum tryptase level > 20 ng/ml and the presence of aberrant mast cell morphology and surface markers of CD2 and CD25. Medical work-up is in accordance with standard medical practice. Mastocytosis patients will be children and adults from two years to 80 years of age. The protocol is designated for up to a 1-year enrollment period; with only a small number of enrolled patients that will stay on study for more than one visit, based on investigator assessment of contribution to study objectives. Patients may be asked to re-enter this protocol at a later time for further research or entry into protocol 98-I-0027. This is not a therapeutic protocol. Treatment using FDA licensed/approved drugs may be provided to patients on a case-by-case basis at the discretion of the principal investigator. and This protocol does not involve infusion of any manipulated cells, viruses or DNA constructs into human subjects.