Immune thrombocytopenia (ITP) is a frequently encountered disease in childhood. Recent reports pointed to the benefit of high-close steroid in ITP treatment since it resulted in a better outcome in a shorter time than TV immunoglobulin therapy. In the authors' clinic, mainly after 1984, megadose methyl prednisolone (MDMP) has been used for ITP treatment. There is no report that includes an extensive immune system, examination of megadose steroid effect in childhood ITP. The purpose of this study is to determine the effects of MDMP therapy on the immune system in childhood ITP for serum immunoglobulins, absolute lymphocyte and lymphocyte subclass counts, and in vitro blastogenic responses to mitogens. The authors have demonstrated that lymphocyte subtypes (CD3,CD4, CD8, CD19) and serum immunoglobulin G, A, M increased after a short-course MDMP therapy in comparison to pretreatment values in mixed group (acute+chronic) of childhood ITP patients. Also blastic transformation junction of lymphocytes with Conca-A and phytohemaglutinin showed an upward trend in 6 of the 9 patients. Steroid are thought to have a suppressive effect on the immune system but this study suggests that short-course MDMP may not be hazardous to the immune system.