Hydroxylapatite (HA) composites with pure zirconia (ZrO(2)), and 3 and 8% Y(2)O(3) doped ZrO(2) were pressure-less sintered in air and hot isostatically pressed (under 120 MPa gas pressure) at 1100 degrees C for 2 h. The reactions and phase transformations were monitored by X-ray diffraction, thermal analysis, and Raman spectroscopy. HA/pure ZrO(2) composites were not thermally stable in air sintering; HA dissociated into a and 3 tricalcium phosphate while monoclinic ZrO(2) was transformed into tetragonal and cubic phases. No decomposition in HA or phase transformation in ZrO(2) were observed in hydroxylapatite/3% Y(2)O(3) doped ZrO(2) or HA/8% Y(2)O(3) doped ZrO(2) composites. On the other hand, HA and ZrO(2) phases in hot isostatically pressed composites remained stable. The highest densification was found in a composite initially containing 10% monoclinic ZrO(2) among the composites sintered in air. The densification of the composites decreased at lower sintering temperatures and higher ZrO(2) contents upon air-sintering. The HIPped composites were densified to about 99.5% of theoretical densities in all mixing ratios. The reactivity between ZrO(2) and HA was dependent on the amount of air in the sintering environment. Hot isostatic pressing with very limited retained air was proved to be a very convenient method to insure both phase stability and full densification during the production of hydroxylapatite zirconia composites. (c) 2010 Elsevier Ltd and Techna Group S.r.l. All rights reserved.