The spontaneously aneurysm-prone Blotchy mouse has a mutation on the X chromosome resulting in low hepatic copper levels; and copper is an essential cofactor for lysyl oxidase, which catalyzes reactions leading to the cross-linking of collagen and elastin. Population characteristics and family histories of patients with aneurysms suggest that aneurysmal disease may also be sex linked in man. Hepatic copper levels were determined in 13 patients who died with abdominal aortic aneurysms and in 13 control patients selected on the criterion of severe atherosclerotic occlusive disease of the abdominal aorta. Excluding two patients with severe liver disease, the tissue copper level in the patients with aneurysms was only 26% of the control level. The results suggest that additional studies of the biologic markers for aneurysm formation in the Blotchy mouse should be carried out prospectively in human subjects.