With a growing elderly population, the incidence of isolated systolic hypertension (ISH) has increased. This study characterizes dynamic vascular changes that occur with advanced age and with ISH. Fifty-five healthy individuals and seven with ISH were distributed in seven age groups from the second to the seventh decade. An index of aortic stiffness (delta P/delta V) was derived using a mercury sphygmomanometer to obtain pulse pressure, and ultrasonographic measurements were used to estimate aortic volumes applying the "cylinder formula." The mathematic derivation of this formula is explained in detail. Pulse pressure showed no significant change with age, but showed a significant increase with ISH. A decrease in volume change from systole to diastole was found with advanced age. Normotensive subjects aged 65 +/- 2 years had a 2.6-fold increase in aortic stiffness compared with young individuals. Elderly patients with ISH had a 7-fold increase in aortic stiffness compared with Group 1 (15 +/- 2 years) (p < 0.001) and a 2.7-fold increase compared with Group 6 (normotensive subjects aged 65 +/- 2 years). A strong correlation between systolic pressure and arterial stiffness was observed (r = 0.953) (p < 0.001). The proposed stiffness index was compared with the one described by Hirai, obtaining a high correlation, that is, r = 0.989 (p < 0.001). When compared with Stefanadis' index of distensibility, our index showed a correlation of r = 0.932 (p < 0.003). It is concluded that while systolic pressure is a main determinant of arterial stiffness, the delta P/delta V is a more sensitive method to estimate dynamic changes in elastic arteries such as the aorta.