We investigated the genotype frequency of methylenetetrahydrofolate reductase (MTHFR) 1298A>C polymorphism in the group of patients with bipolar disorder type I (BDI) (n=200) and schizophrenia (n=200) and in the control group (n=300). Odds ratio (OR) for patients with BD and schizophrenia in 1298CC homozygous state was 3.768 (95% CI=1.752-8.104); P=0.0003; (P=0.0006 after Bonferroni correction) and 2.694; (95% CI=1.207-6.013); P=0.0123 (P=0.0246 after Bonferroni correction), respectively. The stratification of patients based on gender revealed significant association of 1298CC genotype with female patients only with BDI (OR=7.293; 95% CI=2.017-26.363; P=0.0005). Our results confirm association of BD and schizophrenia with the 1p36.3 MTHFR locus and with the methyl group transfer using folate-dependent one-carbon pathway.