Hyperlipidaemia in children is most commonly expressed as hypercholesterolaemia. "Normal values" for serum cholesterol, if defined statistically, vary between communities, and levels of cholesterol in childhood above which an increased risk of coronary heart disease in adult life may be expected have not been firmly established. It is suggested that serum cholesterol concentration over 250 mg/dl (6.47 mmol/l) in a child over 1 year of age merits detailed investigation, including full lipoprotein analysis, and levels of serum cholesterol between230 and 250 mg/dl (5.95-6.47 mmol/l) should be repeated with further studies if indicated. Secondary hyperlipoproteinaemia rarely presents diagnostic problems but must always be excluded. The only primary hyperlipoproteinaemia likely to be encountered in childhood is familial hyperbetalipoproteinaemia in its common heterozygous form. The most effective means to date of lowering serum cholesterol in this condition is cholestyramine, but the long-term consequences of therapy are not known and treatment should at present be limited to children from high-risk families. Long-term follow-up is essential and until results of such studies are available population screening is unjustified.