Although the long-term clinical results of cemented total hip arthroplasty are quite good, there is a significant problem with failure after approximately 10 years, especially with younger patients. Uncemented total hip arthroplasty has been developed in response to prosthesis loosening due to cement fragmentation and osteolysis. Porous-coated hip stems have been used, with generally promising results. However, clinical studies of porous-coated prostheses have revealed findings of concern such as thigh pain, subsidence of the femoral prosthesis, and radiographic lucencies at the bone-prosthesis interface. Hydroxyapatite coatings were developed to improve the fixation of uncemented prostheses. This article discusses the chemical, mechanical, and biological properties of hydroxyapatite coatings, and reviews the biologic behavior of experimental hydroxyapatite-coated implants. The clinical and radiographic results of a 4-year prospective trial are presented. The authors conclude that the reported experimental evidence, as well as the early clinical experience of this series and others, support the continued clinical evaluation of hydroxyapatite-coated titanium for total hip arthroplasty in younger, active patients.