Objective To investigate the correlations of plasma total homocysteine (tHcy) level and the C677T methylene tetrahydrofolate reductase (MTHFR) polymorphism with vascular dementia (VaD) and Alzheimer's disease (AD).Methods A case control study in 64 AD patients (AD group),72 VaD patients (VaD group) and 56 healthy controls (control group) was under taken.Plasma tHcy level was determined by enzymatic cycling assay.The MTHFR677C/T polymorphism was analyzed by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP).Results tHcy levels were significantly higher in AD and VaD groups than in control subjects(P＜ 0.05).VaD patients also showed increased levels of tHcy as compared to AD patieuts,but no statistic significance was found(P＞0.05).Logistic regression analysis indicated that tHcy level was a risk factor for dementia (including AD and VaD together)[(OR=1.15),VaD (OR=1.23),AD (OR=1.328)].MTHFR677 TT genotype represented higher plasma tHcy concentrations than CC and CT genotypes(both P＜0.05).The distribution frequency of the T allele of MTHFRC677T was 45.3 ％ (58/128)in AD group,49.0％(51/104)in VaD group and 33.0％(37/112)in control group,and it had significant differences between the three groups (P＜0.05).There was a significant difference in distribution frequency of the T allele of MTHFRC677T between VaD and control group (P＜0.01).The distribution frequency of MTHFR677 genotype (CC/CT/TT) had a significantly difference between the VaD group and the control group(P＜0.01).Conclusions High plasma level of tHcy is closely correlated with AD and VaD.tHcy level is the risk factor for dementia (including AD and VaD together).The polymorphism of MTHFR677C/T affects the plasma tHcy level.The T allele of MTHFRC677T may correlate with VaD,while has no direct correlation with AD.