The objective of this study was to compare stentless bioprostheses with stented bioprostheses with regard to the postoperative changes in left ventricular (LV) mass and function. Forty patients with aortic stenosis undergoing valve replacement with a stentless (20 patients) or a stented (20 patients) bioprosthesis were evaluated early (baseline), 1 year, and 2 years after operation. Left ventricular mass index was calculated with the corrected American Society of Echocardiography formula. The relative changes between end-diastole and end-systole in LV mid-wall radius, length, and volume (ejection fraction) were determined with a previously validated model for dynamic geometry of the left ventricle. Overall, a significant decrease was found in LV mass index (from 155 ± 30 to 112 ± 23 g/m²; P < .001) and a significant increase in longitudinal shortening (from 0.12 ± 0.11 to 0.22 ± 0.08; P < .001), and ejection fractions (from 0.67 ± 0.11 to 0.71 ± 0.10; P = .017). No significant change was found in the mid-wall radius shortening fraction. Two years after surgery, the extent of LV mass regression was greater in stentless bioprostheses (–51 ± 18 vs –35 ± 17 g/m²; P = .01), though the average mass index was similar in both groups (114 ± 26 vs 110 ± 20 g/m²). Also at 2 years, the longitudinal shortening fraction was greater in patients with a stentless bioprosthesis (0.25 ± 0.07 vs 0.18 ± 0.08; P = .03). In conclusion, this study suggests that the superior hemodynamic performance of stentless bioprostheses may have some benefits with regard to LV mass regression and function after aortic valve replacement. The significance of these benefits in terms of prognosis remains to be determined. (J Am Soc Echocardiogr 1999;12:981-7.)