Effect of age on the efficacy of blood pressure treatment strategies. To study whether the proportion of excess cardiovascular events attributable to various levels of systolic blood pressure varies with age, we calculated the population-attributable risk of all-cause mortality, fatal and nonfatal cardiovascular events (stroke, coronary heart disease, angina, congestive heart failure, and peripheral vascular disease), and stroke incidence due to systolic blood pressure in men and women 45 years of age or older in the United States during 1980. Our estimates are based on US census counts, blood pressure prevalence distributions from the second National Health and Nutrition Examination Survey, and the annual risk of cardiovascular complications during 18 years of follow-up in the Framingham cohort. We then determined the impact of age on the relative efficacy of mass treatment and case-finding strategies in preventing systolic blood pressure-related events. At 45-54 years of age, only 30-40% of systolic blood pressure-related excess events occur in hypertensive individuals (systolic blood pressure greater than or equal to 160 mm Hg). With increasing age, however, the percentage of systolic blood pressure-related events that occur in hypertensive individuals rose substantially; in the older age group (greater than or equal to 75 years), 65-70% of fatal and nonfatal cardiovascular disease events occur in hypertensive persons. The pattern is similar for men and women. The potential impact of a mass treatment strategy designed to shift the distribution of blood pressure downward by a small amount is greater in younger than in older groups, whereas an opposite trend is seen for a high-risk, hypertensive case-finding and treatment approach. 