Fourteen cases of diaphyseal endosteal erosions adjacent to the femoral stem have been identified in our patient population of 308 primary cementless titanium alloy total hip replacements at minimum two-year follow-up. Three of these cases were associated with unstable prostheses that resulted in revision surgery. Eleven cases were associated with radiographically stable prostheses. one of which was revised. Capsular tissue and interfacial membrane tissue were harvested from the four cases that were revised. All the tissues were analyzed histologically. and selected tissues were analyzed with electron microprobe analysis (EMA). analytic electron microscopy (AEM). and Fourier transform infrared spectroscopy (FTIR). Standard hematoxylin and eosin sections demonstrated histiocytic infiltration of the capsular and interfacial membrane tissue. These cells were laden with fine intracellular birefringent material. EMA revealed the presence of titanium alloy particles and AEM documented titanium alloy particles as well as silicate particles. FTIR positively identified polyethylene-like particles, but this technique was not able to identify particles less than 10 μm in diameter. Femoral endosteal osteolysis appears to be mediated by histiocytes containing both metallic and polymeric wear debris.