 Mastocytosis in infants and children is an unusual disease characterized by an excess of mast cells in tissues. In pediatric onset mastocytosis, disease is usually localized to the skin and disease is considered to be of limited duration. However, a subset of children appear to develop a clinical picture resembling that observed in adults who have adult-onset disease. This study will focus on children with more severe mastocytosis in an attempt to define its pathogenesis, focusing on mutations and polymorphisms in genes regulating mast cell proliferation and survival to determine if they might contribute to this disease pattern. If severe pediatric-onset disease does not fit within the existing classification, new criteria of diagnosis must be proposed for the pediatric age group. 