In conclusion, the MTHFR 677G>A polymorphism was statistically associated with grade 3-4 hematologic toxicity in both the first and second cycles of treatment of Thai colorectal cancer patients who received 5-FU-based therapy, whereas the MTHFR 1298A>C polymorphism had no significant association. The gene discussed is MTHFR; the disease is colorectal cancer.