Three-dimensional laser Doppler velocimetry (LDV) was used to characterize the flow created by the On-X bileaflet mechanical heart valve (MHV) manufactured by Medical Carbon Research Institute (MCRI), Inc. (Austin, TX). The valve was mounted into a pneumatically driven single-shot chamber in the mitral position such that only the closure dynamics were simulated. Measurements taken 2 mm proximal to the valve housing showed a peak velocity of 1.8 m/s and maximum Reynolds Shear Stresses (RSS) of 17,500 dynes/cm2, which were found along the centerline of the valve in the hinge region 2 ms after valve closure. The large velocity and RSS gradients denote the presence of complex flow structures. These results provide an initial basis for understanding the impact of valve geometry on hemolysis and thrombosis associated with the On-X MHV.