The hemodynamic effects of olprinone, a newly synthesized phosphodiesterase (PDE) III inhibitor, were assessed in patients with a large cardiac left-to-right shunt. Ten patients with a large ventricular septal defect (VSD) were evaluated during cardiac catheterization. Olprinone was administered as a bolus, 20 microg/kg body weight, and hemodynamic data were obtained before and after the administration. Heart rate and systemic flow increased significantly after administration. On the other hand, olprinone significantly reduced left and right atrial pressure, the systolic pulmonary/arterial pressure ratio, and systemic vascular resistance. However, pulmonary flow and pulmonary vascular resistance were not changed. These results suggested that olprinone had a positive inotropic effect and selective vasodilator effect on patients with a large ventricular left-to-right shunt. Thus, PDE inhibitors may be beneficial for the treatment of patients with a large VSD.