Hemodynamic studies were performed in 22 patients three to ten months after insertion of cloth-covered Starr-Edwards prostheses. The aortic valve was replaced in ten patients, the mitral in ten, and both valves in two. Moderate aortic systolic pressure gradients were found in all patients with aortic prostheses (range 17 to 68 mm Hg, mean 41 mm Hg), and the calculated valve orifice areas were small (average 0.92 cm 2 ). Mitral diastolic pressure gradients in those with mitral prostheses averaged 9.4 mm Hg and were associated with persistently elevated left atrial pressure (average 18 mm Hg). These findings compare unfavorably with the hemodynamic characteristics of previous Starr-Edwards prostheses. Factors contributing to the functional stenosis appear to be smaller valve orifices and turbulent blood flow generated by the coarse cloth surfaces and a shorter excursion of the ball.