Objectives: The aortic annulus diameter measured by transthoracic echocardiography yields lower values than by computed tomography, and echo-based selection of transcatheter aortic valve prosthesis size has been implied to result in more frequent paravalvular leakage. We investigated the relation of preoperative annulus diameter by echo with the ring size of the aortic prosthesis chosen by direct assessment during open-heart aortic valve replacement. Methods: Preoperative annulus diameter by echo (from parasternal long-axis cross-sections of the left ventricular outflow tract and aortic valve) and implanted prosthetic diameter (tissue annulus diameter, determined intraoperatively using a sizing instrument) were compared retrospectively in 285 consecutive patients undergoing open-heart aortic valve replacement. Results: A total of 285 prostheses (240 biologic and 45 mechanical) were implanted, with prosthetic diameter ranging between 19 and 27 mm. There was a significant linear correlation (P < 0.0001) with r = 0.51, between preoperative annulus diameter by echo (mean 21.8 +/- 2.8 mm) and prosthetic diameter (22.9 +/- 1.7 mm). Preoperative annulus diameter of patients receiving prostheses no. 21, 23 and 25 mm aortic prostheses (the most frequent prosthesis sizes) were significantly different (P < 0.001) from each other. On average, preoperative annulus diameter by echo underestimated prosthetic diameter by a bias of 1.07 mm. Conclusion: Our data confirm that preoperative echo assessment of the aortic valve may slightly underestimates the optimal surgical prosthesis diameter for the aortic valve annulus.