The prevalence of obesity in children has increased dramatically in the United States and across the western world in the last few decades. The causes and consequences of this epidemic are not fully understood, making the formulation of effective preventive policies difficult. The aim of this study is to describe the longitudinal association between changes in objectively measured physical activity, accurately measured fat mass and cardiovascular disease (CVD) risk factors in a large, population based cohort of children followed from before birth to late puberty. This proposal is for a competing continuation of our existing NHLBI project (1R01HL071248-01A1) based within the Avon Longitudinal Study of Parents and Children (ALSPAC). In our current project (2003-07) we have objectively measured levels and patterns of physical activity using accelerometers on 5,595 children at age 11 and linked these data to concurrent Dual Emission X-Ray Absorptiometry (DXA) measures of fat mass. We are have repeated the physical activity and fat mass measurements on the whole cohort at age 13 and are currently checking and cleaning these data. From our analyses to date we have demonstrated that physical activity levels in contemporary children are low, with most children, especially girls, failing to meet current recommended levels for health. Further, we have identified a strong, inverse dose-response association between level of physical activity and risk of obesity. In this renewal, we aim to confrim and clarify this association prospectively by following children through to age 15. We propose to repeat the physical activity measurements and link these data with further measurements of fat mass (separately funded), blood pressure and a range of CVD risk factors from a fasting blood sample (CVD risk factors are: insulin, glucose and lipids)(also seperately funded). These data will enable us to further explore the longitudinal associations between physical activity and obesity during this critical period of puberty and to extend these analyses to examine associations with CVD risk factors. This proposal, which extend our database from before birth through to the age of 15, will be uniquely placed to explore the role of physical activity in the development of obesity and CVD risk factors in contemporary children. Our results will inform the design of intervention studies and public health policies to prevent the development of obesity and CVD risk factors in children.