Somatic mutations in the GATA1 gene are present in almost all cases of Down syndrome (DS)–associated acute megakaryoblastic leukemia (AMKL) and transient leukemia (TL). An in utero origin of the GATA1 mutation suggests it is an early leukemogenic event. To determine the detectable incidence and clinical relevance of GATA1 mutations in DS newborns, we screened Guthrie cards from 590 DS infants for mutations in the GATA1 gene. Twenty-two (3.8%) of 585 evaluable infants harbored a predicted functional GATA1 mutation; 2 were identified exclusively within intron 1. Hispanic newborns were 2.6 times more likely to have a mutated GATA1 gene than non-Hispanics (P = .02). Two newborns with a GATA1 mutation subsequently developed AMKL, and none of the infants without a functional GATA1 mutation were reported to have developed leukemia. In addition to screening for TL, a GATA1 mutation at birth might serve as a biomarker for an increased risk of DS-related AMKL.