In 1955, efforts began to identify twins who had served in the Armed Forces during World War II. Copies of birth certificates of white male twins born in the years 1917-1927 were matched against Department of Veterans Affairs (VA, then the Veterans Administration) files to determine veteran status. Some 54,000 pairs of birth certificates were obtained from 42 vital statistics offices, and eventually 15,924 of these pairs were identified by the VA as veterans. These 31,848 individual twins constitute the NAS-NRC Twin Registry. Over the years, many research studies, ranging over a wide variety of medical topics, have been undertaken using the Registry.