The crystalline structure of Zirconia femoral heads provides for superior fracture toughness when compared with alumina. Transformation of the crystalline structure that takes place with time in service may produce changes in the surface and biomechanical properties of these implants. This study examines surface and mechanical property changes of Zirconia femoral heads that occur with time in situ. Eighteen retrieved Zirconia femoral heads were compared to 5 factory-sealed controls. The retrieved implants demonstrated significant transformation to a monoclinic phase. This phase transformation was associated with decreased surface hardness. There was evidence of increased surface roughness with increasing time of implantation. The phase transformation that takes place in Zirconia femoral heads may render these implants less desirable as a bearing surface in total hip arthroplasty. © 2004 Elsevier Inc. All rights reserved.