In 1966, the National Academy of Sciences-National Research Council (NAS-NRC) began the development of a twin registry. Through state birth records, a group of twins born in 1917-1927 was ascertained and matched to veteran records to obtain a roster of 16,000 twins where both members of the pair were in military service. Veterans formed the basis of the registry because Veterans Administration files provided access to current addresses as well as potential for mortality follow-up. From this roster, a group of twins living within 200 miles of the examining centers and agreeing to cooperate was examined. This procedure yielded a set of 514 pairs of twins where both members of the pair were examined. At the time of initial examination, the World War II veteran twins were 42-56 years old. Since the initial examination, the twins have been followed by the NAS-NRC to maintain current addresses and mortality information.