Infants with platelet counts below 100,000/mm3 should be evaluated for the cause of the thrombocytopenia. A maternal history to determine previous illnesses and particularly thrombocytopenia, drugs taken, and perinatal complications is important, and a maternal platelet count must be obtained. Physical examination of the infant is revealing in the TAR and giant cavernous hemangioma syndromes and may suggest intrauterine infection. A complete blood cell count (CBC) should be done to look for associated anemia and neutropenia or for polycythemia. Red cell morphology may be helpful. A bone marrow examination may be necessary if the etiology is unclear after the initial studies are done. Investigation of the well child will usually find an etiology for the thrombocytopenia. It is important to consider and test for isoimmune thrombocytopenia and intrauterine infection. In the ill infant multiple factors may contribute to a low platelet count, and a single, precise etiology often cannot be established. Thrombocytopenia with or without DIC may be an important indicator of sepsis. Platelet transfusions are helpful if the thrombocytopenia is due to decreased production or intrinsic platelet defects. In disorders with increased platelet destruction, donor platelets may survive long enough to be of some benefit. In ill infants treatment of the underlying problem often results in resolution of the thrombocytopenia. Transfusions should be used for the bleeding child and for the infant with severe thrombocytopenia who is the product of a vaginal delivery. In addition to being therapeutic, platelet transfusions may also assist in diagnosis.