Cardiovascular disease remains a major cause of ill-health and death. Maximum aerobic capacity (VO2p), a measure of circulatory capacity, is a strong predictor of future cardiovascular (CV) disease and general health. Most inter-individual differences in VO2p are established by age 30 and VO2p subsequently declines with age. The factors responsible for differences in peak circulatory capacity at maturity are not understood and we will identify them by studying participants in the Avon Longitudinal Study of Parents and Children (ALSPAC), a birth cohort of over 15,000 individuals who have undergone detailed studies since before birth. This study will provide a unique insight into the determinants of peak circulatory capacity in adults and identify potentially modifiable factors for improvement of future cardiovascular health.