An inactivating polymorphism at position 609 in the NAD(P)H:quinone oxidoreductase 1 gene (NQO1 C609T) is associated with an increased risk of adult leukemia. A small British study suggested that NQO1 C609T was associated with an increased risk of infant leukemias with MLL translocations, especially infant acute lymphoblastic leukemia (ALL) with t(4;11). We explored NQO1 C609T as a genetic risk factor in 39 pediatric de novo and 18 pediatric treatment-related leukemias with MLL translocations in the United States. Children with de novo B-lineage ALL without MLL translocations and a calculation of the expected genotype distribution in an ethnically matched population of disease-free subjects served as the comparison groups. Patients with de novo leukemias with MLL translocations were significantly more likely to be heterozygous at NQO1 C609T (odds ratio [OR] = 2.77, 95% confidence intervals [CI] 1.17-6.57; P =.02), and significantly more likely to have low/null NQO1 activity than patients with de novo B-lineage ALL without MLL translocations (OR = 2.47, 95% CI 1.08-5.68; P =.033). They were also significantly more likely to have low/null NQO1 activity than expected in an ethnically matched population of disease-free subjects (OR = 2.50, P =.02). Infants younger than 12 months old at diagnosis of leukemia with t(4;11) were most likely to have low/null NQO1 activity (OR > 10.0). Conversely, the distribution of NQO1 genotypes among patients with treatment-related leukemias with MLL translocations was not statistically different than in the comparison groups. The inactivating NQO1 polymorphism is associated with an increased risk of de novo leukemia with MLL translocations in infants and children.