We have developed a pneumatic type ventricular assist device (VAD) for pediatric use and implanted it in six mongrel dogs to study its efficacy and to identify any problems. The diaphragm-type blood pump has a stroke volume of 34 cc. The ventricle of the blood pump was made from polyurethane to enhance antithrombogenesis. The pump was implanted between the left atrium and the descending thoracic aorta and it functioned in a fill-to-empty control mode. The average pump flow rate was 0.41 L/min when the pump rate was 60 bpm. The duration of support ranged from 1 to 26 hours. The hemodynamic effects of the VAD on the heart failure model were monitored. An increase in pump flow rate was indicated in such a deteriorated heart failure and respiratory failure. Studies to date suggest that, with further refinement, a more reliable VAD with clinical applications could be developed.