The need for a bone marrow examination in all children with suspected idiopathic thrombocytopenic purpura (ITP) has been challenged in recent years. By definition, ITP is a diagnosis of exclusion, which means that all known entities that can cause thrombocytopenia have been excluded by a clinical assessment that includes a history, physical examination, and appropriate laboratory tests. There are many diseases that, because of their typical clinical manifestations, do not rely on a bone marrow examination for determining the cause of the thrombocytopenia. Examples include hypersplenism, systemic lupus erythematosus, disseminated intravascular coagulation, septicemia, hemolytic-uremic syndrome, and familial thrombocytopenia. Since children with ITP have a typical presentation and course, why have we insisted on bone marrow examinations as part of our routine? After all, the bone marrow morphologic findings are nondiagnostic in ITP. However, it helps subdivide the patients with thrombocytopenia into those with adequate to increased megakaryocytes and those with