Objective:To study the metabolism abnormality of homocysteine(Hcy) in unexplained repeated spontaneous abortion(URSA).Methods:The level of Hcy in sera was measured with hyperpressure liquid chromatography(HPLC);Folic acid and vitamin B12 were detected by radioimmune assay;anticardiolipin antibody (ACA) was detected by ELISA.Results:(1)The level of serum Hcy in URSA group was significantly higher than that in control group,showing a statistical significant difference(P＜0.01).The level of Hcy was correlated with ages,but not correlated with areas,numbers of miscarriage,gestation age,primary or secondary abortions.(2)The levels of folic acid and vitamin B12 in URSA group were significantly lower than those in control group.The levels of serum folic acid and vitamin B12 were not correlated with age,area,numbers of miscarriage and abortion periods.(3)ACA positive rate in URSA was significantly higher than that in control group.The level of Hcy in ACA(+) group was significantly higher than that in ACA(-) group among URSA patients.Conclusions:Hyperhomocysteinemia,low folic acid state,and ACA were all the independent risk factors for URSA.Lacking of folic acid and vitamin B12 is one of the important causes of hyperhomocysteinemia.ACA and hyperhomocysteinemia may have synergistic action in the occurrence of URSA.