To reduce the possible thrombogenicity of the pump studied, pump characteristics and washout conditions were compared between a pump with a semi-open and a pump with a full-open impeller. A difference in hydrodynamic performance was observed between the semi-open impeller and the full-open impeller; the pressure in the former was less by approximately 10%, and the maximum attainable efficiency decreased from 0.41 to 0.34. The flow pattern, as visualized by the oil film method, showed that the washout condition was enhanced by addition of the shroud, especially at the bottom region of the pump where the blood flow tended to be stagnant. The stagnant area was observed in the suction side of the impeller in both models, where the vortices shed from the impeller tip contributed to the washout. It was also shown that the flow entering the bottom region was circumferentially uniform in the full-open impeller, whereas in the semi-open impeller the flow was not uniform and entered primarily from the vicinity of the outlet port. The semi-open impeller, thus, was demonstrated to have better washout conditions than the full-open impeller regardless of a slight decrease in hydrodynamic efficiency.