The effect of mitral valve replacement on left ventricular volume, mass, function and clinical symptoms was examined in 20 patients with chronic mitral regurgitation. Pre- and postoperative echocardiograms demonstrated that two outcomes could be defined. Left ventricular dimension at end-diastole was reduced to normal postoperatively in the 16 Group I patients, but was unchanged in the 4 Group II patients. The Group I response was associated with a dramatic reduction in left ventricular mass and a decrease in clinical symptoms; all 16 patients reached New York Heart Association functional class I. No change in left ventricular mass was seen in the Group II patients and all four remained symptomatic despite continued medical therapy. Examination of the preoperative echocardiographic measurements indicates that these data have significant prognostic value in predicting surgical outcome. When ventricular dimension at end-systole exceeds 2.6 cm/m2 or fractional shortening is less than 31% or end-systolic wall stress index exceeds 195 mm Hg, all Group II patients are identified and there are no false positive results in Group I. It is concluded that echocardiographic measurements of left ventricular size, function and wall stress can provide important prognostic information in patients with chronic mitral regurgitation. Such data may allow improved patient selection and a better definition of an optimal time for mitral valve replacement.