1. Aortic compliance and plasma lipid and lipoprotein levels were measured in 20 young patients with heterozygous familial hypercholesterolaemia and in 20 age- and sex-matched control subjects. 2. Patients with familial hypercholesterolaemia had significantly higher plasma cholesterol, low-density lipoprotein-cholesterol and triacylglycerol levels than control subjects (P < 0.001, P < 0.001 and P < 0.005, respectively). The patients with familial hypercholesterolemia also had significantly more compliant (distensible) aortas than the control subjects (P < 0.001), a significant inverse correlation being observed between compliance and age (r = 0.73, P < 0.001) and between compliance and mean blood pressure (r = -0.60, P < 0.005). 3. When the effects of age and sex on aortic compliance were corrected for, the blood pressure effect disappeared, significant correlations being observed between normalized compliance and cholesterol (r = 0.50, P < 0.03), low-density lipoprotein-cholesterol (r = 0.54, P < 0.01), high-density lipoprotein-cholesterol (r = -0.44, P < 0.05), low-density lipoprotein-/high-density lipoprotein-cholesterol ratio (r = 0.60, P < 0.0006) and duration of disease (r = 0.67, P < 0.002). Multivariate regression analysis showed that the low-density lipoprotein-/high-density lipoprotein-cholesterol ratio (P < 0.03) and duration of disease (P < 0.04) were the best predictors of normalized compliance. 4. We suggest that the measurement of aortic compliance in young patients with familial hypercholesterolaemia may potentially be a useful, non-invasive, research tool for assessing their susceptibility to atheroma.