The 5,10-methylenetetrahydrofolate reductase (MTHFR) is a key enzyme for intracellular folate homeostasis and metabolism. Two common MTHFR polymorphisms, C677T and A1298C, which lead to an altered amino acid sequence, have been associated with a decreased enzyme activity and susceptibility to cancer suggesting that these genetic variants may modulate the risk of several malignancies. C667T, and to a lesser extent A1298C polymorphisms, are also reported to influence the cytotoxic effect of fluoropyrimidines and antifolates providing support for their pharmacogenetic role in predicting the efficacy and the toxicity in cancer and rheumatoid arthritis patients. A combined polymorphisms and haplotype analysis may result in a more effective approach than a single polymorphism one. Moreover gene-nutrient/environmental and gene-racial/ethnic interactions have been shown to affect the impact of these MTHFR genetic variants. Further well-designed studies are needed to clarify the role of MTHFR polymorphisms to derive dose adjustment recommendations on the basis of the patient's genotype.