Transient myeloproliferative disorder (TMD) occurs in 10% of infants with Down syndrome (DS). Down syndrome infants with resolved TMD may later develop acute megakaryocytic leukemia (AMKL). In these patients, AMKL is associated with somatic mutations in the X-linked transcription factor gene, GATA1. AMKL also has been described after TMD in children without DS. We report on a non-DS child identified with trisomy 21 mosaicism and a GATA1 mutation in the original blast cells who has been followed for 2 years without exhibiting AMKL. Currently, the risk for such infants developing acute leukemia is uncertain. We recommend that nondysmorphic infants with TMD undergo chromosome analysis for trisomy 21 and testing for GATA1 mutations to aid surveillance for leukemic transformation.