Stroke is a leading cause of morbidity and mortality in North America. Primary prevention of stroke includes lifestyle modifications and measures to control blood pressure, cholesterol levels, diabetes mellitus, and atrial fibrillation. Lowering blood pressure in patients with hypertension prevents both hemorrhagic and ischemic stroke (relative risk reduction, 35 to 45 percent). Observational studies suggest that higher cholesterol levels are associated with an increased risk of ischemic stroke, and treatment with statins (3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors) may reduce the risk of fatal and nonfatal stroke by 25 percent. Although high-quality evidence linking tighter glucose control with stroke reduction is lacking, good glucose control and aggressive treatment of hypertension and hyperlipidemia in patients with diabetes mellitus are recommended. The risk of stroke in patients with atrial fibrillation and the role of anticoagulation depend on factors such as age and the presence of comorbid conditions. Controversy exists about the roles of angiotensin-converting enzyme inhibitors and aspirin in the primary prevention of stroke.