Objective To investigate the relationship of plasma homocysteine (Hcy) levels and the gene polymorphisms of N5,N10-methylenetetrahydrofolate reductase (MTHFR), cystathionine 3-synthase (CBS) with Alzheimers disease (AD).Methods Plasma Hcy levels were measured by means of high voltage capillary electrophoresis with ultra-violet detection, the polymorphisms of C677T in exon 4 of MTHFR gene and 844ins68 in exon 8 of CBS gene were analyzed by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) in 105 AD patients and 102 non-AD controls. All controls were excluded from cardiocerebrovascular disorders and other diseases.Results The plasma Hcy level in AD patients (16.04 ± 3.84 μmol/L) was significantly higher than that in the controls (11.94±3.87 μmol/L, P＜0.001). There were no significant differences of the genotype and allele frequencies of MTHFR C677T mutation and CBS 844ins68 mutation between the patients and controls. However, the T allele of MTHFR gene was found to relate with the plasma Hcy level increase in all subjects.Conclusion The elevated plasma Hcy level in AD patients is probably involved in the pathogenesis of AD, which may be due to the environmental factor rather than genetic factors of the mutations of MTHFR and CBS.