This protocol is designed to examine those growth potentiating and inhibiting factors which regulate mast cell number in patients with mastocytosis, and to explore the molecular basis of the disease process in hopes of improving therapy. Patients entered into the study are seen initially and may elect to be reevaluated yearly. The majority of patients to be entered into this protocol are currently followed at NIH on other protocols. Medical workup and treatment are in accordance with standard medical practice. The protocol is for 3 years, to be extended as facilities, faculty, and patients permit.