The design parameters of the natural aortic valve in vivo were not known, which may explain why various bioprosthetic valves have been designed differently. The design of the aortic valve was studied in vivo by placing radiopaque markers in the valve. The marker movement revealed that, during a cardiac cycle, the design parameters of the valve were changing continuously with changing aortic pressure and ventricular geometry. During diastole decreasing radius of the commissures (Rc) and increasing radius of the bases (Rb) caused the leaflets to tilt toward the ventricle, thereby decreasing the bottom surface angle (alpha) and increasing the free-edge angle (phi) of the leaflet. During systole Rc increased, Rb decreased, and interleaflet distance decreased, causing a change in the geometry of the open valve from conical to cylindrical. In middiastole the design parameters were Rb/Rc = 1.2, H/Rc = 1.4, phi = 34 degrees, and alpha = 20 degrees, where H is sinus height. How a significant deviation from the design could compromise the efficiency and longevity of the valve is discussed.