This study will examine growth factors that promote and inhibit mast cell proliferation resulting in mastocytosis, a disease of excessive mast cells in the body. These cells can release chemicals that cause itching, blisters, flushing, bone pain and abdominal pain. Patients up to 80 years of age with mastocytosis may be eligible for this 1-day study. Participants will have one visit at NIH lasting up to 8 hours, during which they will undergo the following tests and procedures: * Medical history and physical examination. * Laboratory studies, if medically indicated. * Blood tests to identify genetic changes important in the growth, development, and functioning of mast cells. * Bone marrow aspiration and biopsy. For the bone marrow procedure, the skin over the hipbone and the outer surface of the bone itself are numbed with local anesthesia. Then, a special needle is inserted into the hipbone and about 1 tablespoon of bone marrow is drawn into a syringe. Another needle is inserted into the same area to collect a small piece of the bone marrow. Additional procedures may include allergen testing, urinalysis, and 24-hour urine collection. Participants will receive an evaluation of their mastocytosis. ...