There are some cases in which pump function of the left ventricle (LV) is not sufficiently improved despite of successful operation, especially in advanced hypertrophy. The impaired pump function is believed to result from irreversible myocardial damage. Thus, the present study is designed to make clear the histopathological changes of the LV muscle in hypertrophied hearts. Fifty-six patients with volume-overloaded hearts (27 with AR, 24 with MR, and 4 with AR and MR) and 10 patients with AS were examined. Cardiac muscle specimens for histological examination were obtained from the LV anterior wall. In volume-overloaded hearts, the muscle cell diameters were correlated with both LV dimension (LVDd and LVDs) and LV volume index (LVEDVI and LVESVI). The progress of interstitial fibrosis was correlated with LV volume index. Five of the 9 patients with massive myocardial hypertrophy (in all cases, muscle cell diameter was greater than 30 microns) did not show any improvement in histological figure after operation and displayed persistent postoperative LV dilatation. Patients with either LVDd greater than 75mm and LVDs greater than 55mm or LVEDVI greater than 200ml/m2 and LVESVI greater than 100ml/m2, whose hearts showed massive myocardial hypertrophy and severe fibrosis, tended to have postoperative cardiac dysfunction. In pressure-overloaded hearts, the muscle cell diameters were correlated with LVPWTh and LVESVI. Noteworthily, fibrosis and degeneration of muscle fibers were slight as compared with advanced myocardial hypertrophy.