Background Dyslipidemia is a primary risk factor for the development of atherosclerosis. Aortic distensibility is an important determinant of left ventricular function and coronary blood flow whose possible alterations in patients with dyslipidemia have not been fully investigated. Methods To assess the effect of dyslipidemia on the elastic properties of the aorta, we studied 60 patients (mean age 37 ± 11 years) with heterozygous familial hypercholesterolemia and no manifest arterial disease and compared them with 20 of their normolipidemic siblings (mean age 34 ± 10 years). Two indexes of the aortic elastic properties were measured: aortic distensibility was calculated by use of the formula: 2 × (AoS-AoD)/PP × AoD, and aortic stiffness index was calculated by use of the formula: ln (SBP/DBP)/(AoS-AoD)/AoD, where AoS and AoD are aortic root end-systolic and end-diastolic diameters, respectively, SBP and DBP are systolic and diastolic arterial pressure, respectively, and PP is pulse pressure. Internal aortic root diameters were measured at 3 cm above the aortic valve by use of two-dimensional guided M-mode transthoracic echocardiography, and arterial pressure was measured simultaneously at the brachial artery by sphygmomanometry. Results The mean aortic systolic and diastolic diameter index did not differ signifficantly between the two groups. In contrast, aortic distensibility was found to be significantly reduced in subjects with isolated familial hypercholesterolemia compared with that in the control group (2.15 ± 1.72 cm² · dynes^-1 · 10^-6vs 3.18 ± 1.58 cm² · dynes^-1 · 10^-6 p < 0.02). In addition, the mean aortic stiffness index was double in patients with familial hypercholesterolemia compared with that in normolipidemic subjects. Conclusions Severe dyslipidemia does not overtly influence aortic dimensions but leads to impairment of aortic elastic properties before the occurrence of clinical manifestations of atherosclerotic disease. (Am Heart J 1998;135:604-8.)