To assess the role of treated diastolic blood pressure (DBP) level in stroke, coronary heart disease (CHD), and cardiovascular disease (CVD) in patients with isolated systolic hypertension (ISH).An analysis of the 4736 participants in the Systolic Hypertension in the Elderly Program (SHEP) was undertaken. The SHEP was a randomized multicenter double-blind outpatient clinical trial of the impact of treating ISH in men and women aged 60 years and older.Cox proportional hazards regression analysis, with DBP and systolic blood pressure (SBP) as time-dependent covariables.After adjustment for the baseline risk factors of race (black vs other), sex, use of antihypertensive medication before the study, a composite variable (diabetes, previous heart attack, or stroke), age, and smoking history (ever vs never) and adjustment for the SBP as a time-dependent variable, we found, for the active treatment group only, that a decrease of 5 mm Hg in DBP increased the risk for stroke (relative risk, [RR], 1.14; 95% confidence interval [CI], 1.05-1.22), for CHD (RR, 1.08; 95% CI, 1.00-1.16), and for CVD (RR, 1.11; 95% CI, 1.05-1.16).Some patients with ISH may be treated to a level that uncovers subclinical disease, and some may be overtreated. Further studies need to determine whether excessively low DBP can be prevented by more careful titration of antihypertensive therapy while maintaining SBP control. It is reassuring that patients receiving treatment for ISH never perform worse than patients receiving placebo in terms of CVD events.