The factor II G20210A mutation is a recently identified congenital risk factor for venous thrombosis. Its role in artery disease is still undefined. We investigated 72 patients (35 male and 37 female) with documented ischemic stroke occurred before 50 years of age and without risk factors such as diabetes, hypertension, and hyperlipidemia; 198 thrombosis-free individuals were investigated as the control group. We found 7 heterozygotes (9.7%) and 2 homozygotes (2.7%) for the mutant factor II allele among the patients and 5 heterozygotes (2.5%) among the controls; the mutant factor II allele frequency in the patient group (7.6%, 95% confidence interval [CI], 3.3 to 11.9) was significantly higher than in the controls (1.2%; 95% CI, 0.1 to 2.3; P = .0001). The prevalence of other investigated mutant alleles (factor V G1691A, methylenetetrahydrofolate reductase C677T) did not significantly differ between the two groups. The odds ratio for ischemic stroke associated with the carriership of the mutant factor II allele (both heterozygous and homozygous genotypes) was 5.1 (95% CI, 1.6 to 16.3). Heterozygous genotype was associated with a 3.8-fold increased risk for cerebral ischemia (95% CI, 1.1 to 13.1); in particular, assuming an expected prevalence of homozygotes in the general population of 1.6 to 10,000 according to the Hardy-Weinberg equilibrium, the risk associated with the homozygous genotype was estimated exceedingly high, being increased 208-fold.