The National Heart, Lung and Blood Institute (NHLBI) twin study is a collaborative, longitudinal study of the role of genetic risk factors on subsequent cardiovascular disease processes in 514 pairs of white, male World War II and Korean veteran twins born between 1917 and 1927. This paper describes the sampling procedures and zygosity determination at the initial examination, participation by the cohort members at later examinations, and a summary of the variables provided for Genetic Analysis Workshop 8 (GAW8).