Stent creep, often associated with valve malfunction, is said to play an important role in the long-term performance of a porcine bioprosthesis. We have measured the angle of the stent post showing maximal inward bending (IBA) on 44 mitral porcine bioprostheses. All of them were explanted 1 to 12 years (mean explant time, 7.8 years) postoperatively at reoperation. Patients included 19 men and 25 women, ranging in age from 24 to 66 years (mean age, 47.2 years). Mean IBA was 12.7 +/- 4.2 [SD] degrees in 14 valves implanted for 7 years or less and 16.4 +/- 5.8 degrees in 30 valves implanted 8 years or longer (p less than 0.05). There was no significant difference in IBA among valves based on type (25 Hancock valves, 15.0 +/- 4.7 degrees; 10 Angell-Shiley valves, 16.2 +/- 8.6 degrees; and 9 Carpentier-Edwards valves, 14.4 +/- 3.7 degrees). There was a significant difference in IBA based on valve size (37 valves measuring 25-29 mm in diameter, 16.3 +/- 5.1 degrees; and 7 valves measuring 31 mm in diameter, 9.9 +/- 5.1 degrees; p less than 0.05). IBA showed a tendency to have a large value in a heart with a small left ventricular end-systolic volume. We conclude that (1) stent creep is not related to the materials or designs of the stent post, but tends to increase with passage of time in place; and (2) compression of the left ventricular wall is one of the main causes of stent-post bending.