How did the study come about? Ecological studies conducted in the 1980s demonstrated a close geographic correlation between death rates from coronary heart disease during the years 1968–78 in different parts of England and Wales and the infant mortality rate in these areas 60 years earlier.1 These studies suggested that adverse environmental influences acting in utero and during infancy might increase the risk of cardiovascular disease in later life. However, this hypothesis required investigation using more robust epidemiological techniques. To yield results within a relatively short space of time, a cohort study was needed which linked information about the early environment of individuals born at least 60 years earlier to their health outcomes in later life. As part of a nationwide search of archives, staff working at the MRC Environmental Epidemiology Unit (MRC EEU), University of Southampton, discovered a large set of records maintained in Hertfordshire during the early 20th century.