Serum concentrations of total cholesterol and of HDL cholesterol have been measured in 170 Irish children, aged 1–12 years, attending hospital for conditions other than hyperlipidaemia. There were no significant differences between the sexes, or between fasting and non-fasting samples for either estimation. Concentrations of total cholesterol were estimated using a specific enzyme assay and gave a mean value of 3.8 mmol/L±0.8 (one S.D.) (146 mg/100 ml±30). These values are less than those reported in adults using the same method. There was no variation with age. In contrast, sedum concentration of HDL cholesterol increased with age (1.01 mmol/L±0.04 X age in years) (40 mg/100 ml+1.7 X age in years), r=0.4; reaching adult values at the age of 7 years. These data provide background information for studies of lipid disorders in children.