Elevated homocysteine concentration is known to be related to placental abruption, spontaneous abortion, and many adverse pregnancy outcomes. The purpose of this study was to investigate the effects of folic acid supplementation ($1000{cal}ug$ per day) and 5, 10 methylenetetrahydrofolate reductase (MTHFR) C677T polymorphism genotype on serum homocysteine and B vitamin levels in 50 infertile women ($31.2{pm}3.2;years$). Blood sampling was performed at baseline and at the end of folic acid supplementation period. In infertile women, serum folate and vitamin $B_{12}$ concentrations were significantly higher in post-supplementation than those in pre-supplementation. Serum homocysteine concentration was significantly lower in post-supplementation than that in pre-supplementation. However, serum homocysteine levels were still higher in the T/T genotype than those in the C/C or C/T even after folic acid supplementation. Serum homo-cysteine was inversely related to serum folate in T/T homozygotes at baseline and at the end of folic acid supplementation. These results suggest that folic acid supplementation is needed for infertile women to improve their vitamin status and also to reduce the risk of hyperhomocysteinemia. These effects were different according to their MTHFR C677T genotypes. Therefore, further studies are necessary to determine the optimal level of supplementation of folic acid by MTHFR genotypes.