Control of hypertension, labile or fixed, systolic or diastolic, at any age, in either sex appears to be central to prevention of atherothrombotic brain infarction (ABI). Prospectively, hypertension proved the most common and potent precursor of ABI's. Its contribution was direct and could not be attributed to factors related both to stroke and hypertension. Asymptomatic, causal "hypertension" was associated with a risk of ABI about four times that of normotensives. The probability of occurrence of an ABI was predicted no better with both blood pressure measurements or the mean arterial pressure than with systolic alone. Since there was no diminishing impact of systolic pressure with advancing age, the concept that systolic elevations are, even in the aged, innocuous is premature. Comparing normotensives and hypertensives in each sex, women did not tolerate hypertension better than men.