Samples of the middle cerebral artery (MCA) and the brachial artery were obtained post mortem from 14 patients who died following rupture of intracranial saccular aneurysms and from a control group of 14 age- and sex-matched patients who died of causes unrelated to aneurysm rupture. The biomechanical properties of ring-shaped arterial specimens were investigated by loading the specimens at a constant deformation rate until rupture. The relative amounts of collagen type I and type III were determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) studies of cyanogen bromide peptides of collagen prepared from the arterial samples. A deficiency of collagen type III was demonstrated in specimens of the MCA in six of 14 patients with a ruptured intracranial saccular aneurysm. This deficiency was not accompanied by alterations in the mechanical arterial strength but resulted in a significant increase in the extensibility at stress values corresponding to blood pressures between 100 and 200 mm Hg. No difference was found between aneurysm patients and the control group in regard to the biomechanical properties of the brachial artery, despite the presence of a significant deficiency of collagen type III. The increase in vascular extensibility of the MCA may represent alterations in the fibrous structure and functional integrity of the cerebral arteries of aneurysm patients with collagen type III deficiency. Together with aggravating hemodynamic stresses, this deficiency may be an important factor in the pathogenesis of saccular aneurysms.