Background Recent research indicates that levels of conventional coronary heart disease risk factors in children are related to the premature development of atheroma. It is therefore important to determine how risk factors might be modified on a population scale in children.Methods and Results In 1985, the Australian Schools Health and Fitness Survey was conducted on a representative sample of Australian schoolchildren aged 7 to 15 years. In children aged 9, 12, and 15, data on plasma cholesterol, triglycerides, and high-density lipoprotein cholesterol were obtained along with measurements of blood pressure, fitness, and body fatness. From an original sample of 2400 in these three age categories 1919 underwent the full set of measurements. Univariate analysis of these data revealed a strong association between body fatness and plasma lipids. There was no significant association between fitness (measured as physical work capacity at a heart rate of 170 beats per minute per kilogram of lean body mass) and plasma lipids, but a significant negative association was found for fitness and systolic blood pressure (r=-.12, P<.001). Multiple regression analysis revealed that the association of fitness with systolic blood pressure was only partly accounted for by the confounding effect of lower body fatness in fitter children.Conclusions These data collected on a representative sample of children under standardized conditions confirm a previous finding of a link between fitness and blood pressure in schoolchildren and also support a growing consensus that fitness is only weakly linked to plasma lipids and lipoproteins in children and adolescents. (Circulation. 1994;89:1539-1544.)