Knowledge of the biomechanical behavior of abdominal aortic aneurysm (AAA) as compared to nonaneurysmal aorta may provide information on the natural history of this disease. We have performed uniaxial tensile testing of excised human aneurysmal and nonaneurysmal abdominal aortic specimens. A new mathematical model that conforms to the fibrous structure of the vascular tissue was used to quantify the measured elastic response. We determined for each specimen the yield (sigma y) and ultimate (sigma u) strengths, the separate contribution to total tissue stiffness by elastin (EE) and collagen (EC) fibers, and a collagen recruitment parameter (A), which is a measure of the tortuosity of the collagen fibers. There was no significant difference in any of these mechanical properties between longitudinal and circumferential AAA specimens, nor in EE and EC between longitudinally oriented aneurysmal and normal specimens. A, sigma y, and sigma u were all significantly higher for the normal than for the aneurysmal group: A = 0.223 +/- 0.046 versus A = 0.091 +/- 0.009 (mean +/- SEM; p < 0.0005), sigma y = 121.0 +/- 32.8 N/cm2 versus sigma y = 65.2 +/- 9.5 N/cm2 (p < 0.05), and sigma u = 201.4 +/- 39.4 N/cm2 versus sigma u = 86.4 +/- 10.2 N/cm2 (p < 0.0005), respectively. Our findings suggest that the AAA tissue is isotropic with respect to these mechanical properties. The observed difference in A between aneurysmal and normal aorta may be due to the complete recruitment and loading of collagen fibers at lower extensions in the former. Our data indicate that AAA rupture may be related to a reduction in tensile strength and that the biomechanical properties of AAA should be considered in assessing the severity of an individual aneurysm.