To determine the optimum time for surgical intervention in pure mitral regurgitation, 39 patients were studied by echocardiography before and after surgery. Pure mitral regurgitation included mitral valve prolapse with or without ruptured chordae tendineae. Twenty-two patients had mitral valve replacement and 17 had mitral valve reconstruction. Serial echocardiograms were obtained from all 39 patients (average follow-up: 40 months). Two of the 39 patients died suddenly at home on 26 and 62 months after surgery. One of the 39 patients now suffers from congestive heart failure. These three patients had preoperative left ventricular end-diastolic dimensions greater than 70 mm and left ventricular end-systolic dimension greater than 50 mm. Operation is recommended for patients with end-diastolic dimension less than 70 mm and end-systolic dimension less than 50 mm.