A pneumatic pediatric ventricular assist device (VAD) with a stroke volume of 20 ml has been developed to treat post-operative heart failure (HF), and maintain transplant candidates. The polyurethane VAD has two #21 Bjork-Shiley valves and the internal diameter of the cannula is either 6 or 8 mm. Hemodynamic effects of a left ventricular assist device (LVAD) on a HF after Fontan's procedure model, and those of a right ventricular assist device (RVAD) on right HF with pulmonary hypertension model, were investigated in acute experiments with four and five dogs, respectively. In the former, the pressure gradient across the lung and cardiac output (CO) increased with an LVAD; right atrial pressure decreased and CO increased with an RVAD in the latter. The pump was implanted as an LVAD in 8 young goats, 9-22 kg in weight, for 4-10 weeks and favorable in vivo performance was demonstrated. The VAD system was applied as an LVAD to two postcardiotomy patients, a 12 kg boy with a ventricular septal defect, and a 13 kg boy with an endocardial cushion defect. In these cases, CO was well maintained at the level of 2.5-4.1 L/min/m2 for three and seven days, respectively, and the pump was removed. In conclusion, this VAD will become a promising circulatory support system for pediatric uses.