A brief review of the pathophysiology of aortic and mitral valve disease and the hemodynamic results of valve replacement with caged ball prostheses are described. In most patients intracardiac pressures and restored to normal at rest, although there are small pressure gradients across mechanical valves. Severe pulmonary hypertension, if present, usually will regress. With exercise, abnormalities of left atrial pressure or left ventricular function may be found after valve replacement. The causes of failure to achieve hemodynamic improvement with surgery and the late return of congestive failure are discussed.