Abstract： Objective To investigate whether genetic polymorphisrns of methylenetetrahydrofolate reductase (MTHFR) contribute to the risk of Down syndrome(DS). Methods Altogether, 100 Chinese mothers who had given birth to DS babies (study group) and 100 matched mothers (control) were chosen and all were Han nationality. Genotypes of MTHFR 677 and MTHFR 1298 were determined by PCR-RFLP and MGB-Taqman resl-time PCR. The concentration of plasma homocysteine (HCY) was measured by chemilumineaence. Results The mutations of one or both alleles of MTHFR 677 and MTHFR 1298 were associated with a 2. 37-fold(95%CI: 1.19～3.05) and 1.97-fold(95%CI: 1.04～3.75) increase in the risk of having a child with DS. Combination of MTHFR 677CT and 1298AC/CC genotype had a 5. 62-fold increased risk of having a child with DS(95%Ch 1.86～17.03) and the combination of MTHFR 677TT and MTHFR 1298AC/CC genotype was significantly associated with an increased risk of DS(OR=11.84, 95%CI: 1.39～100.62). The mean plasma HCY concentration was significantly higher in the study group than that of control [(9.04±3.85) μmol/L vs (6.53±2. 06) μmol/L, P<0.01)]. The presence of 677C→T substitution in one or both alleles was associated with increased plasma HCY in both groups(P<0.05).However, MTHFR 1298A/C mutation alone did not significantly change the level of HCY in neither groups (P>0.05). Higher HCY level was found in the genotype-pairs of MTHFR 677CT/TT and 1298AC/CC (P<0.05). Conclusions Maternal genetic polymorphisms of homocysteine/folate pathway is the risk factor for DS fetus in Han nationality women in China, and a genetic synergistic effect could not be excluded.