1510 plasma cholesterol estimations were made in 1391 children admitted to hospital as part of a biochemical profile. Babies under 1 year and children known to have familial hyperlipoproteinaemia were excluded. The mean concentration was 4-28 mmol/l +/- 1-04 (1 SD) (165-3 mg/100 ml +/- 38-6), and levels exceeded 5-93 mmol/l (229 mg/100 ml) in 68 children. Repeat estimations on 55 of these children showed 34 still to have values greater than 5-93 mmol/l and family studies were performed in 19 of these. In 8 children hypercholesterolaemia was secondary and no familial lipoprotein disorder was present. Familial hyper-beta-lipoproteinaemia (FH) was diagnosed in 3 children and in 2 of the families there was a history of early ischaemic heart disease. In 2 children the diagnosis was in doubt. In the remaining 6 children FH and secondary hyperlipoproteinaemia were excluded so the hypercholesterolaemia was presumably environmentally induced, possibly in association with polygenic inheritance. In the present state of knowledge screening of the childhood population for FH by means of plasma cholesterol determinations cannot be recommended. Studies of lipoproteins should, however, be made in children from families known to have FH or early coronary heart disease.