Abnormalities in left ventricular (LV) structure and function have been shown in patients with diastolic hypertension and recently in subjects with isolated systolic hypertension. The purpose of this study was to determine whether abnormalities of cardiac structure or function are present in elderly subjects with borderline isolated systolic hypertension (defined as systolic blood pressure [BP] between 140 and 159 mm Hg, and diastolic BP < 90 mm Hg). Ninety-one subjects (mean age 77 years) from the original Framingham Heart Study with untreated borderline isolated systolic hypertension, who were free of cardiovascular disease, were compared with 139 normotensive (BP < 140/90 mm Hg) subjects (mean age 76 years). Measurements included M-mode values for LV structure, and 6 Doppler indexes of LV diastolic filling. Subjects with borderline isolated systolic hypertension and the control group differed in mean systolic (147 vs 125 mm Hg) and diastolic (76 vs 70 mm Hg) BP. Borderline systolic hypertension was the most frequent form of untreated hypertension in this elderly group. The sum of LV wall thicknesses (septum+posterior wall) was significantly higher in borderline hypertensive subjects than in normotensive ones (20.5 vs 19.7 mm; p = 0.002). No difference was detected in LV internal dimension or systolic function. After adjustment for age and other clinical variables, comparisons between the groups revealed significant differences in indexes of Doppler diastolic filling. Peak velocity of early filling, and the ratio of early to late peak velocities were lower in the hypertensive group (40 vs 44 cm/s [p = 0.03] and 0.69 vs 0.76 [p = 0.01], respectively).(ABSTRACT TRUNCATED AT 250 WORDS)