The degree of progressive inward deflection of the stent posts ("stent creep") during function of Hancock standard orifice porcine aortic valve bioprostheses is unknown. The present investigation determines, using a quantitative geometric analysis of clinically removed bioprostheses, the contribution of stent creep to progressive reduction in the outflow orifice area of these valves. Fifty-four Hancock standard orifice porcine aortic valve bioprostheses obtained at reoperation or at autopsy of 50 patients were studied; 47 of these were removed more than 21 months postoperatively. The projected geometric outflow orifice area of each prosthesis was planimetrically measured. To compare prostheses through the entire range of available sizes, this measured area was divided by that of an unimplanted valve of the same size to calculate a ratio designated the "normalized outflow area ratio" (NOAR). Thus, by definition, for all unimplanted standards, NOAR was 1.00. In 7 prostheses in place for less than 1 month, the NOAR was 1.00 +/- 0.03 (mean +/- standard deviation). In 47 prostheses removed 21 to 126 months postoperative, the NOAR was 0.91 +/- 0.08. The relation between NOAR and duration of function determined by linear regression was NOAR = 0.955 - 4.560 X 10(-4) X Duration (in months). The minimum NOAR found was 0.74; the NOAR was less than 0.80 in 4 prostheses (8% of long-term valves). Thus, stent creep is usually not prominent after long-term function of Hancock standard orifice procine bioprostheses and suggests that clinically important progressive reduction of the geometric outflow orifice is infrequent.