The purpose of this study was to evaluate the interface attachment strength and histology of hydroxylapatite (HA) coated and uncoated titanium and cobalt-chromium alloy implants. The canine transcortical plug model was utilized. Four different hydroxylapatite coatings were evaluated. In vitro analysis confirmed that all coatings met FDA guidelines for HA coatings. An unspecified FDA parameter, porosity was found to range from 515%. Mechanical testing of the bone-implant interface demonstrated large variation in the performance of the coatings. However, further evaluation of two of the coatings did not demonstrate variations in mechanical characteristics. The histologic findings confirmed the mechanical testing results. The coatings which demonstrated the best mechanical characteristics had excel- lent bone apposition and uniformity and maintenance of the HA coating at all time periods upon histologic evaluation. Conversely, the coatings which demonstrated inferior mechanical characteristics demonstrated variable amounts of bone apposition and moderate to severe coating degradation and breakup. Cell-mediated osteolysis was observed in regions of severe coating degradation, and particle migration was noted in regions far from the interface. It was hoped that the four coatings would behave similarly as they all met current FDA guidelines. The only parameter which differed significantly among the coatings was coating porosity. Our results indicate that coatings with large porosities were associated with increased coating degradation and poor mechanical performance and osteolysis at the bone-coating interface.