Mechanical debonding of the stem/cement interface has been implicated in the failure process of cemented femoral hip components. The nature of this failure process remains poorly understood due, in part, to limited understanding of how interfacial debonding occurs in response to a wide range of loading conditions. The purpose of this investigation was to determine the fracture toughness of the cobalt-chromium alloy/polymethylmethacrylate interface under mixed-mode loading conditions. The hypothesis was that the critical energy release rate was dependent on the phase angle of the crack tip and that the fracture response would be significantly different for a smooth compared with rough interface surface. A novel in-plane shear test fixture was developed with use of a combination of finite element and experimental fracture-mechanics tests. A wide range (-65-60 degrees) of phase angles was determined with the in-plane shear test and a clamped cantilever-beam test. Sixty experimental tests were performed for cobalt-chromium alloy bars with a plasma-sprayed coating or a precoat of polymethylmethacrylate over a satin-finished surface. For the specimens with the plasma-sprayed coating, critical energy release rates (500-700 J/m2) were not a function of the phase angle of the crack tip. In contrast, critical energy release rates (15-80 J/m2) were found to be strongly affected by the phase angle for the specimens precoated with polymethylmethacrylate. The critical energy release rate for specimens with the plasma-sprayed surface was significantly (p < 0.01) greater than for those precoated with polymethylmethacrylate. The critical energy release rate increased markedly with the phase angle of the crack tip for the specimens precoated with polymethylmethacrylate. The results suggest that the failure response of a stem with a plasma-sprayed surface may be insensitive to the loading angle of the crack tip, whereas a stem precoated with polymethylmethacrylate may be more likely to debond under tensile opening loading.