Obesity in children is on the increase, and is associated with heart enlargement. This could be due to increases in the load on the heart, although abnormal nervous control of the heart or altered arterial function could also contribute. Low birthweight children, who exhibit accelerated growth through childhood, may be particularly affected. We will use non-invasive techniques to examine heart and arterial structure and function in 2500 fifteen year olds who have been part of a detailed study of growth since birth. This will help us understand why obesity results in heart disease, and why adolescents who have grown rapidly in childhood are at particular risk.