A girl with Down's syndrome was born with a myeloproliferative disorder. The child had spontaneous regression of the myeloproliferation, with acute leukemia developing at a later date. Morphologic, cytochemical, immunologic, and immunoglobulin gene configuration studies all supported the diagnosis of acute nonlymphocytic leukemia. High-resolution chromosome studies revealed that the leukemic cells consistently contained a translocation between chromosomes 1 and 19: der(19)t(1;19)(q25;p13). Spontaneous regression of the transient myeloproliferative syndrome of the newborn with Down's syndrome may not always be permanent, and the transient myeloproliferative syndrome may sometimes represent an early sign of acute nonlymphocytic leukemia.