From a study of the evolution of atherothrombotic brain infarction (ABI) in the Framingham cohort of 5,209 men and women over 18 years of follow-up, it has been ascertained that hypertension is the most common and most powerful precursor. Atherothrombotic brain infarction developed in hypertensive patients seven times more often than in normotensive patients, and the risk was proportional to the blood pressure throughout its range. Various components of blood pressure, including systolic and diastolic blood pressure, pulse pressure, lability of pressure, mean arterial pressure and tension-time index, were analyzed in relation to ABI incidence. While all measures were associated with ABI incidence, the simple casual systolic pressure emerged as good a predictor of ABI incidence as any other component of the pressure. The other measures added very little to risk.