Left ventricular function was compared in 15 patients who had conventional mitral valve replacement (Group 1) and 12 patients who had replacement with preservation of chordae tendineae and papillary muscles (Group 2) for isolated chronic mitral regurgitation. Function was assessed by radionuclide ventriculography. The two groups were similar in terms of age, preoperative functional class, valve pathology, and preoperative left ventricular function. There was uniform intraoperative and postoperative management by a single surgeon. All patients received porcine bioprostheses. Left ventricular function was assessed 1 day before and 3 to 6 months after the operation. Left ventricular ejection fraction decreased from 0.55 +/- 0.09 to 0.48 +/- 0.14 in Group 1 (p less than 0.01) and did not change significantly in Group 2 (from 0.53 +/- 0.14 to 0.52 +/- 0.16). Left ventricular end-diastolic volume, end-systolic volume, and stroke volume indices decreased significantly in both groups. During maximal postoperative bicycle exercise with the patient supine, left ventricular ejection fraction and stroke volume index increased significantly in Group 2 only. Cardiac output increased in both groups during exercise, but this increase was due to increased heart rate in Group 1 and to increased heart rate and stroke volume in Group 2. These findings suggest that preservation of chordae tendineae and papillary muscles during mitral valve replacement for chronic mitral regurgitation has a beneficial effect on postoperative left ventricular function.