Serum lipids were measured in children and their parents from 40 families in which the father had a myocardial infarction or coronary heart disease (CHD) before age 40 years. The relationship between physical activity and serum lipid concentrations in the children was also evaluated. Twenty six men had one or more abnormal lipid value (in mmol/l): total venous cholesterol (TVC) > 6.24, triglycerides < 2.55, low density lipoprotein cholesterol (LDL-C) > 4.42, or high density lipoprotein cholesterol (HDL-C) < 0.91. There were 15 spouses with significant hyperlipidaemia (values above). In the 107 children examined, TVC mean (SD) was 4.68 (1.17), triglycerides 1.4 (0.8), LDL-C 3.0 (1.0), and HDL-C 1.18 (0.28). Altogether 42% of the children had significant hyperlipidaemia. No significant correlation was found between the degree of physical activity of the children and their LDL-C and TVC concentrations. However, a significant positive correlation was found between the degree of physical activity and HDL-C and a significant negative one with triglyceride concentrations. It is concluded that screening the progeny of young CHD patients is highly productive in identifying young people at excessive risk for future CHD. The data also suggest that promoting high degrees of activity among these children may have a positive influence on risk factors for adult onset CHD.