Transient myeloproliferative disorder is a form of self-limited leukemia that occurs almost exclusively in neonates with Down syndrome. The authors report an unusual case of a newborn without constitutional trisomy 21 who developed undifferentiated leukemia and subsequently achieved clinical and molecular remission without chemotherapy. Cytogenetics and molecular analysis have shown trisomy 21 and GATA1 mutation restricted to leukemic cells. G-to-T transversion was detected, which is predicted to result in a premature stop codon (c.119G>T; pGlu67X) in diagnosis samples. These findings emphasize that there must be a powerful interaction between GATA1 and trisomy 21 in leukemogenesis process.