Transient myeloproliferative disorder is a condition clinically resembling congenital acute myelogenous leukemia. As in acute leukemias the blast cell population in this disorder may have either normal or abnormal chromosomal complement. Transient myeloproliferative disorder is well recognized in neonates with a complete or mosaic trisomy 21 (Down's syndrome). We report a phenotypically and cytogenetically normal infant with this syndrome in whom only blast cells showed trisomy 21. We postulate that the pathogenesis of the transient myeloproliferative disorder in both Down's syndrome infants and those with a normal chromosomal complement is related to abnormal prenatal production of placental regulatory hemopoietic factors caused by chromosomal defect(s) confined to placental tissues.