The purpose of this study was to (1) evaluate the magnitude and distribution of mechanical stresses on closed leaflets of porcine bioprosthetic valves (PBVs) by using a nonlinear, finite-element model, and (2) determine if a relationship exists between the magnitude of stresses at various sites on the leaflet and the incidence of calcification at those sites. Mechanical stresses were found to be highest near the commissures and lowest at the base of the leaflet. At a pressure of 100 mm Hg applied to the closed valve, the maximal principal normal stress was 160 kPa near the commissures, 140 kPa in the body of the leaflet, 70 kPa at the free margin, and 60 kPa near the base. Similarly, the maximal shear stress, at an applied pressure of 100 mm Hg, was 80 kPa near the commissures, 70 kPa in the body of the leaflet, 50 kPa at the free margin, and 40 kPa near the base. This distribution of mechanical stresses on the PBV leaflet coincided with the incidence of calcification of the various regions of the leaflet. Calcification was found most frequently near the commissures, less frequently in the body of the leaflet and free margin, and least frequently at the base. These observations suggest a possible causative relation between the magnitude of mechanical stresses and the site of calcification of PBV leaflets.