Previous studies have suggested that elastin in porcine aortic valve cusps is responsible for restoring collagen fibers to their original geometry between loading cycles. To investigate the role of elastin further, biaxial loading tests were performed on intact aortic valves before and after elastase treatment. Degradation of elastin caused a significant increase in radial dimensions, with a corresponding decrease in radial extensibility and an increase in radial stiffness. Control experiments, in which the valves were treated with buffer only, confirmed that changes in the mechanical properties of the valves after elastase treatment were due to elastin damage. As many explanted bioprosthetic valves exhibit mechanical characteristics similar to those of the elastase-treated valves, we propose that degradation of elastin initiates the process of prosthetic valve deterioration.