Folates are the vitamin B derivatives that play important roles on cell metabolism. They function in cell proliferation, cell growth, amino acid and purine biosynthesis. Methylenetetrahydrofolate reductase enzyme is crucial for folate metabolism and some variations in the gene that encode the enzyme may cause some problems affecting enzyme kinetics. Absence of folates causes a range of pathologies from neural tube defects to congenital anomalies. Recently, variations in the methylenetetrahydrofolate reductase gene are associated with neurological and psychiatric diseases like Parkinsons, Alzheimers and bipolar diseases and the role of these variations in these diseases were better diagnosed and described. In this review, upon giving brief information about folates, we aim to specify the association of methylenetetrahydrofolate reductase gene with Parkinsons, Alzheimers, bipolar and schizophrenic diseases. [Anadolu Psikiyatri Derg 2013; 14(4.000): 378-382]