The hemodynamic function of the 19 mm and 21 mm St. Jude Medical valve prostheses in the aortic position was evaluated in seven patients who had valve replacement for either severe aortic stenosis or regurgitation an average of 4 months after operation. Cardiac output and simultaneous transvalvular gradient were recorded at rest and during moderate supine arm exercise. The average peak systolic gradient was 14 mm Hg (range 0 to 25 mm Hg) at rest and 32 mm Hg (range 23 to 56 mm Hg) with exercise. The average mean systolic gradient was 20 mm Hg (range 10 to 28 mm Hg) at rest and 38 mm Hg (range 30 to 48 mm Hg) with exercise. The mean effective orifice area was 1.2 cm2 (range 0.9 to 1.4 cm2) at rest and 1.1 cm2 (range of 0.9 to 1.3 cm2) with exercise. Mean cardiac output at rest was 4.4 L/min, and, with exercise, 7.4 L/min. Of the five valves evaluated with root aortography, four demonstrated trivial and one mild aortic insufficiency. We conclude that the St. Jude Medical valve prostheses in the aortic position has favorable hemodynamic function which is particularly evident in the 19 mm and 21 mm sizes when compared with other currently available aortic valve prostheses.