The collagen content of the media of infrarenal aorta has been compared in age- and sex-matched normal aorta and dilated and nondilated atherosclerotic aorta. The proportion of collagen was increased in aneurysmal aorta from 62% to 84% and appears to be the result of preferential elastin degradation. The ratio of type I to type III collagen, estimated from sodium dodecyl sulfate—polyacrylamide gel electrophoresis of specific cyanogenbromide peptides, did not vary significantly from 2:1 in any of the three groups of aortas. There was no evidence of increased collagenase activity in unruptured aneurysmal aorta. Collagenase activity was increased in ruptured aneurysmal aorta but could only be satisfactorily measured after resolution from the endogenous tissue inhibitor of metalloproteinases. We suggest that only limited collagen turnover occurs in the media of abdominal aortic aneurysms before rupture. A subgroup of three patients with a significant family history of aneurysm had lower amounts of type III collagen in the aortic media, suggesting that abnormalities in type III collagen may be one of the genetic factors contributing to familial clustering of aneurysms. (J VASC SURG 1987;6:578-82.)