Many of the established and emerging risk factors that are described in the sections that follow such as arterial hypertension, diabetes mellitus, and hyperlipidemia have both genetic and environmental or behavioral compo- Genome-wide association studies have identified common genetic variants for these risk factors. Studies that assess the effect of the cumulative burden of risk alleles of stroke risk factors, as measured by a so-called genetic risk score, are beginning to emerge. For example, the burden of risk alleles for elevated BP was associated with a modest but significant increase in risk for ICH (OR, 1.11; 95% CI, 1.02 1.21; P=0.01) in 1025 cases and 1247 controls of European Whether a genetic risk score will provide clini- cally useful information beyond that afforded by clinical risk factors remains uncertain. Arguably, estimating genetic risk remains crude because only a few loci influencing stroke risk factors or stroke susceptibility have been identified. Common variants on chromosome 9p21 adjacent to the tumor suppressor genes CDKN2A and CDKN2B, which were initially found to be associated with MI,64 66 have also been associated with large-artery ischemic stroke.67 Common vari- ants on 4q25 and 16q22, adjacent to genes involved in cardiac development (PITX2 and ZFHX3, respectively), which were initially found to be associated with AF,68,69 were subsequently associated with ischemic stroke, particularly cardioembolic Although tests are commercially available for the 9p21, 4q25, and 16q22 risk loci, studies have yet to prove that altering preventive therapies on the basis of genotypes leads to improved patient outcomes. Genome-wide association studies have identified novel genetic variants influencing risk of stroke. 