BACKGROUND: Hyperhomocysteinemia is an independent risk factor for cerebrovascular disease. MTHFR 677TT genotype can induce hyperhomocysteinemia. However, the association between this 677TT genotype and ischemic stroke still remains controversial. This study was undertaken to determine whether MTHFR 677TT genotype was associated with certain subtype of ischemic stroke. METHODS: The case group consisted of 129 patients with ischemic stroke and the control group consisted of 157 healthy individuals. We checked their fasting plasma homocysteine levels and analyzed the C677T mutation in the MTHFR gene. The relative risk of MTHFR 677TT genotype was assessed by odds ratios using multivariate logistic regression. RESULTS: Homocysteine levels in plasma were significantly higher in ischemic stroke patients (10.386.44 mol/L) than in controls (8.002.40) (P<0.05). In small-artery disease (11.366.01), the same result was found (P<0.05). On the other hand, the prevalence of the homozygote mutation was not significantly higher in ischemic stroke patients (20.2%) than in controls (13.4%) (adjusted OR 1.39, 95% CI 0.65 to 2.96). The adjusted OR and 95% CI was 2.59 (1.08 to 6.25) for the TT genotype in patients with small-artery disease compared to controls. The 677TT genotype was increased in small-artery disease compared to large-artery disease (adjusted OR 7.60, 95% CI 1.66 to 34.77). CONCLUSIONS: Our findings suggest that the homozygous C677T mutation in the MTHFR gene is a risk predictor in the subtype of ischemic stroke, such as small-artery disease.