Lynch syndrome is caused by germ-line mutations in the DNA mismatch repair (MMR) genes; mutation carriers are predisposed to a variety of cancers, most commonly colorectal and endometrial. The median age of colorectal cancer onset is 45 years and the lifetime risk is approximately 80%, but the onset age varies substantially. It is likely that other low-penetrance genes and environmental factors act as modifiers of the risk associated with the highly penetrant MMR gene mutations. Methylenetetrahydrofolate reductase plays a key role in folate metabolism. We investigated the association of C677T and A1298C, two common polymorphisms in the methylenetetrahydrofolate reductase gene, with risk for early onset colorectal cancer in Lynch syndrome. Subjects were 185 non-Hispanic whites with confirmed DNA MMR mutations. Kaplan-Meier estimates for the age at colorectal cancer onset according to C677T genotypes were significantly different for the CT and TT genotypes compared with the wild-type CC (P = 0.014, log-rank test; P = 0.004, trend test). The median ages at onset were 39 [corrected] years for the CC genotype and 43 [corrected] years for the combined CT and TT [corrected] genotypes and the CT+TT [corrected] genotypes were associated with a reduced age-associated risk for developing colorectal cancer (hazard ratio, 0.55; 95% confidence interval, 0.36-0.85). No differences in ages at onset or risk were found for the A1298C genotypes. This is the first report to our knowledge to provide evidence that the C677T polymorphism modifies the age at onset of colorectal cancer in Caucasian Lynch syndrome subjects with the 677T allele having a protective effect.