Degradation of yttria-stabilized zirconia femoral heads in vivo has been linked to increased roughening and even fracture of the femoral head. To determine whether magnesia-stabilized zirconia is better suited to resist degradation, we characterized the monoclinic phase concentration, surface topography, and microhardness of retrieved zirconia femoral heads. From previous work, we expected yttria-stabilized zirconia heads to undergo considerable tetragonal-to-monoclinic phase transformation in vivo, leading to considerably increased roughness and decreased microhardness, whereas magnesia-stabilized zirconia heads would not experience phase transformation and thus would not roughen or exhibit decreased microhardness. We studied seven yttria-stabilized zirconia and 12 magnesia-stabilized zirconia femoral heads. Yttria-stabilized zirconia heads explanted after 5 years exhibited a rough orange peel-like surface under light microscopy and were rougher than magnesia-stabilized zirconia heads (average roughness approximately 20 nm versus 7.5 nm, respectively), likely attributable to increased mono-clinic phase transformation (approximately 37% by weight) caused by low-temperature aging. The microhardness of yttria-stabilized zirconia heads decreased with age, but the relationship was not noteworthy. In contrast, magnesia-stabilized zirconia retrievals showed no change in monoclinic phase concentration, surface roughness, or microhardness with age. The properties of the yttria-stabilized zirconia evaluated in our study deteriorated in vivo, whereas magnesia-stabilized zirconia did not degrade and appears to be a superior biomaterial for bearing in total joint arthroplasty.