The contribution and limitation of hydroxyapatite coatings to implant fixation are discussed following a 5-year histomorphological study of the bony incorporation of macroporous stemless hemiarthroplasties in dogs. Implants without coating, with a pure titanium coating and with a hydroxyapatite coating were compared. The mean extent of direct contact between bone and hydroxyapatite-coated implants was 66% compared with 0 to 5% in the other implants. Areas of resorption of the coating were seen in every case associated with direct bone apposition on the metal surface. When the initial fixation is stable, osteointegration of hydroxyapatite-coated implants is obtained consistently, despite design features known to preclude bone incorporation. Implant fixation must depend on a mechanical interlock with bone because of the resorbability and mechanical properties of hydroxyapatite coatings.