In flow through cardiovascular implants, hemolysis, and thrombosis may be initiated by nonphysiological shear stress on blood elements. To enhance understanding of the small-scale flow structures that stimulate cellular responses, and ultimately to design devices for reduced blood damage, it is necessary to study the flow-field at high spatial and temporal resolution. In this work, we investigate flow in the reverse leakage jet from the hinge of a bileaflet mechanical heart valve (BMHV). Scaled-up model hinges are employed, enabling measurement of the flow-field at effective spatial resolution of 167 μm and temporal resolution of 594 μs using two-component particle image velocimetry (PIV). High-velocity jets were observed at the hinge outflow, with time-average velocity up to 5.7 m/s, higher than reported in previous literature. Mean viscous shear stress is up to 60 Pa. For the first time, strongly unsteady flow has been observed in the leakage jet. Peak instantaneous shear stress is up to 120 Pa, twice as high as the average value. These high-resolution measurements identify the hinge leakage jet as a region of very high fluctuating shear stress which is likely to be thrombogenic and should be an important target for future design improvement.