Background: Abdominal aortic aneurysms (AAA) are at risk of rupture when the internal load (blood pressure) exceeds the aneurysm wall strength. Generally, the maximal diameter of the aneurysm is used as a predictor of rupture; however, biomechanical properties may be a better predictor than the maximal diameter. Compliance and distensibility are two biomechanical properties that can be determined from the pressure-volume relationship of the aneurysm. This study determined the compliance and distensibility of the AAA by simultaneous instantaneous pressure and volume measurements; as a secondary goal, the influence of direct and indirect pressure measurements was compared. Methods: Ten men (aged 73.6 ± 6.4 years) with an infrarenal AAA were studied. Three-dimensional balanced turbo field echo (3D B-TFE) images were acquired with noncontrast-enhanced magnetic resonance imaging (MRI) for the aortic region proximal to the renal arteries until just beyond the bifurcation. Volume changes were extracted from the electrocardiogram-triggered 3D B-TFE MRI images using dedicated prototype software. Pressure was measured simultaneously within the AAA using a fluid-filled pigtail catheter. Noninvasive brachial cuff measurements were also acquired before and after the imaging sequence simultaneously with the invasive pressure measurement to investigate agreement between the techniques. Compliance was calculated as the slope of the best linear fit through the pressure volume data points. Distensibility was calculated by dividing the compliance by the diastolic aneurysmal volume. Young's moduli were estimated from the compliance data. Results: The AAA maximal diameter was 5.8 ± 0.6 cm. A strong linear relation between the pressure and volume data was found. Distensibility was 1.8 ± 0.7 × 10-3 kPa-1. Average compliance was 0.31 ± 0.15 mL/kPa with accompanying estimates for Young's moduli of 9.0 ± 2.5 MPa. Brachial cuff measurements demonstrated an underestimation of 5% for systolic (P <.001) and an overestimation of 12% for diastolic blood pressure (P <.001) compared with the pressure measured within the aneurysm. Conclusion: Distensibility and compliance of the wall of the aneurysm were determined in humans by simultaneous intra-aneurysmal pressure and volume measurements. A strong linear relationship existed between the intra-aneurysmal pressure and the volume change of the AAA. Brachial cuff measurements were significantly different compared with invasive intra-aneurysmal measurements. Consequently, no absolute distensibility values can be determined noninvasively. However, because of a constant and predictable difference between directly and indirectly derived blood pressures, MRI-based monitoring of aneurysmal distensibility may serve the online rupture risk during follow-up of aneurysms. © 2008 The Society for Vascular Surgery.