AIM: To evaluate joint effects of Methy/entetra-hydrofolate reductase (MTHFR) C677Tgenotypes, and serum folate/vitamin B12 concentrations on promoter methylation of tumor-associated genes among Iranian colorectal cancer patients.METHODS: We examined the associations between MTHFR C677T genotype, and promoter methylation of P16, Hmlh1, and Hmsh2 tumor-related genes amonq 151 sporadic colorectal cancer patients. The promoter methylation of tumor-related genes was determined by methylation-specific PCR. Eighty six patients from whom fresh tumor samples were obtained and 81 controls were also examined for serum folate and vitamin B12, concentrations by a commercia radioimmunoassay kit.RESULTS: We found 29.1% of cases had tumors with at least one methylated gene promoter. In case-case comparison, we did not find a significant association between methylation in tumors and any single genotype. However, in comparison to controls with the CC genotype, an increased risk of tumor methylation was associated with the CT genotype (OR=2.5;95% CI,1.1-5.6). In case-case comparisons, folate/vitamin B12 levels were positively associated with tumor methylation. Adjusted odds ratios for tumor methylation in cases with high (above median) versus low (below median) serum folate/vitamin B12 levels were 4.9 (95% CI,1.4-17.7), and 3.9 (95% CI,1.1-13.9), respectively. The frequency of methylated tumors was significantly higher in high methyl donor than low methyl donor group, especially in those with MTHFR CT (P=0.01), and CT/TT (P=0.002) genotypes, but not in those with the CC genotype (P=1.0).CONCLUSION: We conclude that high concentrations of serum folate/vitamin B12 levels are associated with the risk of promoter methylation in tumor-specific genes, and this relationship is modified by MTHFR C677T genotypes.