Transient myeloproliferative disease (TMD) is often associated with a trisomy 21 cell line, but it is not always associated with clinical signs of Down syndrome. We report on a phenotypically normal newborn boy who presented with a high white blood cell count, undifferentiated blasts, and cutaneous leukemic infiltrates and compare this patient with the literature on TMD and trisomy 21. Chromosome analysis of bone marrow, and subsequently of skin fibroblasts, documented constitutional mosaicism for trisomy 21. A decrease in the frequency of blast cells paralleled a decrease in cells demonstrating trisomy 21 in hematopoietic tissues, and a complete clinical recovery was seen without the use of chemotherapy. Recognition of this transient form of congenital leukemia is important to prevent the unnecessary use of toxic chemotherapeutic agents in such patients.