The NHLBI has made extensive use of the World War II veteran twin registry beginning in 1969, when the NHLBI Twin Study was initiated. At that time, NHLBI used the World War II veteran twin registry to recruit a group of 514 pairs of twins in five geographic areas to participate in a study which included a battery of questionnaires and a physical examination. Since that time, a second examination and third examination of the cohort have been completed and the registry has been used to maintain contact with the twins and provide surveillance for morbidity and mortality events of the participants in the study.