Experiments were carried out to test the hypothesis that familial aortic aneurysms, either thoracic or abdominal, are caused by mutations in the gene for type III procollagen (COL3A1) similar to mutations in the same gene that have been shown to cause rupture of aorta and other disastrous consequences in the rare genetic disorder known as Ehlers-Danlos syndrome type IV. A family was identified through a 37-yr-old female captain in the United States Air Force who was scrutinized only because many of her direct blood relatives had died of ruptured aortic aneurysms. The woman was heterozygous for a single-base mutation that converted the codon for glycine 619 of the alpha 1(III) chain of type III procollagen to a codon for arginine. Studies on cultured skin fibroblasts demonstrated the mutation caused synthesis of type III procollagen that had a decreased temperature for thermal unfolding of the protein. The same mutation was identified in DNA extracted from pathologic specimens from her mother who had died at the age of 34 and a maternal aunt who died at the age of 55 of aortic aneurysms. Examination of DNA from samples of saliva revealed that the woman's daughter, her son, a brother, and an aunt also had the mutation. The results demonstrated that mutations in the type III procollagen gene can cause familial aortic aneurysms and that DNA tests for such mutations can identify individuals at risk for aneurysms.