Preventing stroke by the modification of risk factors. Epidemiologic research has revealed the major risk factors in cerebrovascular disease. This review will concentrate on three important risk factors: elevated blood pressure, the most common and important, since it is responsible for up to 70% of all strokes; raised cholesterol; and smoking. These factors are important not only because they increase the risk of stroke, but also because they are amenable to modification by drugs, diet, or other interventions. Strategies to avoid stroke can either 1) try to produce substantial reductions, usually with drugs, in the level of the risk factor in the few individuals in the population with high levels (the "high-risk" approach), or 2) try to produce modest reductions in the level of the risk factor in every individual in the population, usually not with drugs but with lifestyle modification (the "mass" approach). The prevention of stroke could best be achieved through continuing medical efforts to deal with high-risk individuals and through political strategies to encourage a healthier lifestyle in the population as a whole. 