A heart valve prosthesis and method of implanting the prosthesis are disclosed. A valve is mounted within a support apparatus that is deformable between a first condition and a second condition. The prosthesis has a cross-sectional dimension in the second condition that is less than a cross-sectional dimension of the supported valve when in first condition. The prosthesis can be implanted into a patient's heart, such as during a direct vision procedure through a tubular implantation apparatus that maintains the prosthesis in its second condition until discharged from the tubular apparatus.