Modes of failure of Hancock and Carpentier-Edwards (C-E) porcine bioprosthetic valves placed in the mitral position are not completely understood. We reviewed transesophageal echocardiographic (n = 19) and pathologic features of failed Hancock (n = 22) and C-E (n = 8) porcine mitral valves in 30 patients (mean age 70 +/- 13 years). Age at implantation (59 +/- 14 vs 58 +/-14 years, p = 0.9), time to implanted valve degeneration (13 +/- 5 vs 11 +/- 2 years, p = 0.3), and size of bioprosthesis (30 +/- 2 vs 31 +/- 2 mm, p = 0.14) of the implanted Hancock and C-E valves were similar. Anterior leaflet was flail in 15 versus flail posterior leaflet in 5 patients (p = 0.0004). Eccentric posterior mitral regurgitation jet was present in 12, eccentric anterior jet in 2, central jet in 2, and paravalvular jet in 3 patients. Stenosis of bioprosthesis was present in 1 1 Hancock versus 1 C-E valve (p = 0.06). Stent creep at any stent post was present in 14 Hancock versus no C-E valve (p = 0.0013). Large commissural dehiscence was present in 5 C-E versus 1 Hancock valve (p = 0.0006). Ring margin perforation was the most common perforation in Hancock valves (p <0.05, analysis of variance versus all other Hancock perforations). Dehiscence at the stent posts was the most common perforation in C-E valves (p <0.05 vs other C-E perforations, analysis of variance and p <0.001 versus Hancock valves). Thus, Hancock valves showed greater stenosis and stent creep, whereas C-E valves showed large dehiscences at the stent posts on explantation. The anterior leaflet degenerated most frequently in both valves. These findings suggest that the valve design may influence the mechanisms of porcine valve degeneration.