Objective. To investigate the relationship of plasma homocysteine (Hcy) level to stroke and genetic factor to elevated plasma Hcy level.Methods. The plasma Hcy level was measured by capillary electrophoresis- ultraviolet detection and the gene polymorphism of N5, N10 - methylenetetrahydrofolate reductase (MTHFR) was studied with PCR - RFLP assay in 43 patients with cortical cerebral infarction and 42 healthy controls.Results. The plasma Hcy level of the patients ( 19. 3 + 6. 0 μ mol/L) was markedly higher than that of the controls (13.7 + 5.4 μ mol/L) ( t = 4. 16, P ＜ 0. 001). There are 3 genotypes, C/C, C/T and T/T, about base - variation of MTHFR gene at locus 677. The plasma Hcy level of the subjects with T/T genotype was higher than that of subjects with other genotypes. However, the frequencies of each genotype and allele were not significantly different between the patients and the controls.Conclusions. The elevated plasma Hcy level is a risk factor for atherothrombotic cerebral infarction, and is related to the C→T mutation at locus 677 of MTHFR gene.