Patients with familial hypercholesterolaemia have severe coronary-artery disease early in adult life. Whether lipid-lowering treatment should be started in childhood remains to be established. We therefore assessed 201 children heterozygous for familial hypercholesterolaemia and 80 unaffected siblings (both age ranges 8-18 years) with B-mode ultrasound to measure carotid wall intima-media thickness. Mean combined carotid intima-media thickness of heterozygotes was significantly greater than that of unaffected siblings (0.494 mm [SD 0.051] vs 0.472 [SD 0.049], p=0.002). A significant deviation in intima-media thickness was noted from age 12 years in children with familial hypercholesterolaemia. Findings on multivariate analysis showed LDL cholesterol, age, and sex to be strong and independent predictors of intima-media thickness. Since raised LDL cholesterol concentrations can be lowered efficiently, clinical studies are needed to investigate long-term safety and effectiveness of statin treatment in children with familial hypercholesterolaemia.