The sewing cuff of the ATS Medical valve is made from double velour polyester, making the sewing cuff diameter larger than the tissue anulus diameter of this prosthetic heart valve. The ATS valve sizer, with the diameter marked in millimeters, has a ring which allows for an easy measurement of the patient's tissue anulus. However, the diameters of this ring and the prosthetic valve (tissue anulus diameter) are made to equal in size. Therefore, if the valve size is determined by using this sizer, the sewing cuff diameter will be considerably larger than the patient's tissue anulus diameter. In order to also take into account the diameter of the sewing cuff, we added a flange around the sizer in the shape of the sewing cuff. The new and more practical sizer allows us to avoid sizing trouble in valve replacement surgery.