The registry of World War II veteran twins came into existence in the late 1960's through a search and link effort of state vital statistics records and Veterans Administration files through the Medical Follow-up Agency of NAS-NRC. Since that time, the registry has served as a data resource for investigators in a variety of scientific fields who want to study genetic and environmental aspects of various human characteristics. Because of the size and geographic coverage throughout the United States, the registry offers a method of recruiting sizeable numbers of twins for study even under geographic restrictions. Of special value to medical researchers is the built-in medical follow-up for morbidity and mortality through Veterans Administration insurance and benefits records. This allows researchers to gather baseline data which may be related prospectively to morbidity and mortality events.