 This study will examine children and young adults age birth to 21 years with pediatric-onset disease previously enrolled on NIAID mastocytosis protocols, or as a result of physician referral, which have disease more consistent with adult-onset disease or parameters associated with increased morbidity. The evaluation may include serum tryptase, blood count, bone marrow biopsy and aspirate, morphology and mutational analysis. Subjects may be asked to return in 12-24 months for assessment. If relevant mutation effecting mast cell growth and function are identified, such mutations will be sought in the biologic parents as appropriate, but only when such mutations are believed to be germ line not somatic mutations. This study will aid in the understanding of the characteristics of severe mastocytosis in the pediatric age group. 