Functional variants in the methylenetetrahydrofolate reductase (MTHFR) gene, including the 677C>T and 1298A>C polymorphisms, have been associated with a moderately reduced risk of several cancers, including colorectal cancers. While recent studies have investigated the role of these polymorphisms on bladder cancer susceptibility, results have been mixed. To clarify the role of MTHFR polymorphisms on bladder cancer risk, we genotyped MTHFR 677C > T and MTHFR 1298A > C in a population-based study of bladder cancer of 352 patients and 551 controls from New Hampshire, USA. The allelic frequency was 35.6% for MTHFR 677C>T and 40.4% for MTHFR 1298A > C among controls. We found no evidence of a main gene effect for either polymorphism (adjusted OR for MTHFR 677C>T variants versus the reference genotype = 1.1; 95% CI, 0.8-1.4 and adjusted OR for MTHFR 1298A>C variants versus the reference genotype = 1.0; 95% CI, 0.7-1.4). Odds ratios did not appear to differ by smoking status or gender. We observed differences in the risk estimates for the MTHFR polymorphisms by arsenic exposure, but they were not statistically significant (P = 0.67 for MTHFR 677C > T and P = 0.12 for MTHFR 1298A>C). Thus, our findings do not support the presence of a main gene effect. The possibility that MTHFR polymorphism affects susceptibility to environmental exposures warrants further consideration.