Patients 18 years of age and older who have episodes of anaphylaxis or flushing with no apparent cause or who have mastocytosis may be eligible for this study. Participants will have a medical history and physical examination; blood tests to identify genetic changes that are important in the growth, development, and functioning of human mast cells; and 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.