Previous studies have demonstrated that left ventricular assist device (LVAD) implantation significantly decreases myocyte size and reduces fibrosis of the left ventricle (LV). The objectives of the present study were to evaluate LV functional recovery after LVAD implantation and to assess its predictive factors, including histological findings of LV. Six patients with idiopathic cardiomyopathy underwent LVAD support with an EVAHEART implantable centrifugal pump (Sun Medical Technology Research, Nagano, Japan) for an average support duration of 2.91 years. Histologic samples were obtained from their LV apexes at the time of implantation. At 1 month and at 24 months after implantation, brain natriuretic peptide (BNP) and echocardiographic parameters were evaluated. Brain natriuretic peptide values, LV end-diastolic dimension, LV end-systolic dimension, functional shortening, and right ventricular systolic pressure (RVSP) were improved after LVAD implantation. Patients with developing fibrosis had longer durations of heart-failure history and higher pulmonary artery pressures. Patients with hypertrophic myocytes had smaller FS preoperatively. There was a correlation between the amount of fibrosis and the rate of BNP value change after LVAD implantation. In patients with less fibrosis and smaller myocytes preoperatively, improvement in LV function was observed during LVAD support.