Objective : To analyze the interrelationship between homocysteine and methylenetetrahydrofolate reductase (MTHFR) mutation in patients with recurrent spontaneous abortion. Material and Method: Homocysteine and MTHFR mutation were tested by fluorescent polarizing immunoassay and PCR-RFLP method, respectively. Results: In patients with homocysteine level less than 5 ?mol/L, there was no case of normal group but there were four cases of heterozygosity and one case of homozygosity. In patients with homocysteine level 5$sim$10 ? mol/L, the number of normal, heterozygosity and homozygosity group were eleven, eighteen and eight, respectively. In patients with homocysteine level $10{sim}15$ ? mol/L, the number of normal, heterozygosity and homozygosity group were four, one and one, respectively. In patients with homocysteine level more than 15 ? mol/L, there was no case of normal and heterozygosity group but there were two cases of homozygosity. Conclusions: Hyperhomocysteinemia due to MTHFR mutation is a cause of recurrent spontaneous abortion. And there was a significant relationship between homocysteine and MTHFR mutation.