Thrombus formation on the aortic face of the disc and tissue overgrowth along the sewing ring adjacent to minor outflow region are major clinical pathologic findings observed with the present model Björk-Shiley aortic prosthesis. These pathologic findings may be attributed to the stagnation zone observed near the aortic face of the disc, and the low flow and shear in the minor outflow region of the prosthesis. In hope of reducing these pathologic conditions the new convexo-concave Björk-Shiley prosthesis was developed. In vitro velocity measurements made with a laser-Doppler anemometer in the immediate downstream vicinity of the convexo-concave Björk-Shiley aortic valve indicate that the design changes have decreased the size of the stagnation zone and have increased flow and shear in the minor outflow region. We conclude that the problems of thrombus formation and tissue overgrowth may be reduced in the new convexo-concave Björk-Shiley aortic valve.