['Small prosthesis size has been associated with poorer postoperative outcomes in aortic valve replacement (AVR). We hypothesized that the use of small aortic valve (AV) prostheses does not independently increase operative mortality following AVR, but that mortality may instead be related to comorbidities.', 'We examined the mortality among 4,621 patients who underwent primary AVR operations at 13 different statewide centers from 2003 to 2008. Patients were stratified by prosthesis size into groups with small (≤21 mm, n=1,810) and standard AV prostheses (≥23 mm, n=2,811). The effect of prosthesis size on outcomes was evaluated with univariate and multivariable regression analyses.', 'Operative mortality among patients undergoing primary AVR operations was 3.7%. Among isolated operations, small AV prostheses were implanted in more females (79.9% vs 21.0%, p<0.001) and older patients (68.9±12.3 years vs 63.8±13.9 years, p<0.001) than were standard-size AV prostheses, and carried a higher predicted risk of mortality according to the Society of Thoracic Surgeons Predicted Risk of Mortality Score (3.1 [interquartile range, 3.0] versus 2.2 [2.0], p<0.001) than did standard-size AV prostheses. Small AV prostheses incurred more major complications (19.5% vs 15.7%, p=0.01), a greater mortality (3.9% vs 2.3%, p=0.03), a longer postoperative length of stay (6.0 [3.0] vs 5.0 [3.0] days, p<0.001), and higher total costs ($29,738 [18,196] vs $26,679 [14,890], p<0.001) than did standard AV prostheses. However, when analyzed with multivariate regression, small AV prosthesis size and female gender were not independent predictors of operative mortality, whereas advanced age, cardiopulmonary bypass time, and aortic annular enlargement were important predictors of operative mortality.'] Does small prosthesis size in aortic valve replacement affect mortality?