Serum lipids and lipoproteins were measured in 67 high-risk children, aged between 2 and 17 years, who were the offspring of 33 fathers with hypercholesterolaemia. 18 of the 33 fathers had had a myocardial infarction (MI) before 44 years of age. In 15 of the fathers there was no history of accelerated coronary heart disease (CHD). No difference in the concentrations of the serum lipids and lipoproteins was found between the children whose fathers had a history of MI (n = 31) and the children with no family history of CHD (n = 36). Serum lipids and lipoproteins were also measured in a control group of 19 children, aged between 2 and 17, whose parents were normolipaemic and had no history of atherosclerosis. Serum total cholesterol (TC) and lowdensity lipoprotein cholesterol (LDLc) concentrations were significantly higher- 5·47 ± 1.36 mmol/l (211 ± 52 mg/100 ml) and 3·55 ± 1·16 mmol/l (137 ± 45 mg/100 ml) respectively-in the 67 'high-risk' children than in the control group-TC, 4·46 ± 0·64 mmol/l (172 ± 25 mg/ 100 ml); LDLc, 2.56 ± 0.64 mmol/l (99 ± 25 mg/100 ml). No statistically significant difference was found in serum triglyceride (TG) or high-density lipoprotein-cholesterol (HDLc) concentrations between these two groups. In 7 of the high-risk children, raised levels of serum TC >5·74 mmol/l (>221 mg/100 ml) were due to raised HDLc >1·95 mmol/l (>75 mg/100 ml) and normal LDLc <3·84 mmol/l (<148 mg/100 ml). In spite of their increased TC levels these children are probably at a lower risk for CHD as a result of the protective effect of HDL. In 3 children, raised concentrations of LDLc (>3·84 mmol/l) were accompanied by low levels of HDLc (<1·19 mmol/l) and normal concentrations of TC (<5·74 mmol/l). These children are probably at the highest risk for CHD. The correct diagnosis would have been missed in 10 (15%) of the children had only TC been measured. It is therefore imperative to measure lipoprotein levels in addition to TC in all children from 'high-risk' families, regardless of their TC concentrations.