We evaluated the association of a polymorphism in TCF7L2 (RS12255372) in the WNT signaling pathway, which previously has been strongly associated with risk of Type II Diabetes, with colorectal cancer (CRC) and adenoma in the prospective Nurses’ Health Study (NHS) and Health Professionals Follow-up Study (HPFS) cohorts. Hyperinsulinemia may be related to the risk of colon adenoma and cancer, therefore this variant associated with reduced insulin secretion would be predicted to be inversely associated with colorectal cancer. Overall, in the NHS and HPFS, there was suggestive evidence for an inverse association associated with homozygosity for the minor allele of RS12255372 (TCF7L2 TT) and CRC (conditional and covariate adjusted OR = 0.63, 95% CI: 0.37–1.08; P for heterogeneity 0.52 for the association in women and men). In summary, the marginal association of the TCF7L2 SNP with CRC might be due to chance, but warrants further laboratory and epidemiological investigation.