Objective To explore the significance of gene mutations of cystathionine ? synthase (CBS) 844ins68,methione synthase (MS) A2756G and methylene tetrahydrofolate reductase (MTHFR) C677T in patients with unexplained repeated spontaneous abortion(URSA). Methods The genotypes of CBS,MS and MTHFR in 50 patients and 56 healthy controls were determined by PCR-based assay and their restricted enzyme digested maps were analyzed. Results Allelic frequency of MTHFR 677C→T mutations in URSA patients was obviously higher than those in controls (24.0% vs 7.1%). The frequency of allelic mutations was elevated in the URSA group comparing with the control (26.0% vs 17.9%). No significant difference was shown in the frequency of CBS and MS mutations between the two groups. Conclusions The MTHFR 677 C→T mutation is associated with URSA. CBS and MS mutations might not be an independent risk factor for URSA.