 This study will evaluate children with mastocytosis, a disease of excessive mast cells in tissues such as skin and bone marrow, to identify the cause of the disease and describe its course. Mast cells can release chemicals that cause itching, blisters, flushing, bone pain, and abdominal pain. Usually, mastocytosis in children involves the skin only and is of limited duration. This study, however, will focus on children with more severe disease that more closely resembles adult-onset mastocytosis. 