ABSTRACTThe hematologic manifestations of neuroblastoma are numerous and varied. Bone marrow invasion by tumor cells may cause leukoerythroblastic changes or depression of one or more of the cell lines in the peripheral blood; occasionally bone marrow involvement may be so extensive that tumor cells may be released into the peripheral blood and lead to an erroneous diagnosis of leukemia. Anemia in neuroblastoma patients may result not only from bone marrow involvement, but also from bleeding into a tumor mass or from the hemolysis accompanying a consumption coagulopathy. A specific morphologic abnormality, the cogwheel erythrocyte, has been reported in patients with neuroblastoma. Neuroblastoma may also be associated with elevation of the platelet count or a hypercoagulable state. Recognition of these protean hematologic manifestations may facilitate diagnosis in children with atypical presentations of this highly malignant tumor.