A significant portion of patients treated for pediatric acute lymphoblastic leukemia (ALL) relapse. We hypothesized that common polymorphisms with moderate effect sizes and large attributive risks could explain an important fraction of ALL relapses. Methylenetetrahydrofolate reductase (MTHFR) is central to folate metabolism and has two common functional polymorphisms (C677T and A1298G). Methotrexate (MTX), which interrupts folate metabolism, is a mainstay of pediatric ALL therapy. MTX inhibits the synthesis of dTMP needed for DNA replication by blocking the conversion of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate by MTHFR. We hypothesized that a deactivating MTHFR allele would increase ALL relapse risk by potentially increasing 5,10-methylenetetrahydrofolate and dTMP, enhancing DNA synthesis and thus opposing MTX. To test this hypothesis, we genotyped 520 patients on the Children's Cancer Study Group ALL study, CCG-1891. The MTHFR C677T variant allele was statistically significantly associated with relapse (chi2 = 4.38, P = 0.036). This association remained significant (hazard ratio = 1.82, P = 0.008), controlling for important covariates, and was more predictive of relapse than other predictors, including day 7 bone marrow response. The MTHFR C677T variant allele was not associated with an increased risk of toxicity or infection. The MTHFR A1298G polymorphism was not associated with altered risks of relapse, toxicity, or infection. Haplotype analysis showed six common haplotypes that did not provide additional information predictive for relapse. These data provide evidence that the MTHFR C677T polymorphism is a common genetic variant conferring a moderate relative risk and a high attributable risk for relapse in pediatric ALL patients.