Sintered composites of hydroxylapatite (HA) and zirconia were studied with X-ray diffraction and scanning electron microscopy (SEM). Addition of zirconia caused increased decomposition of the HA in composites sintered at 1100 degrees C and 1300 degrees C, forming tricalcium phosphate phases (TCP). The volume of the unit cell of HA increased as the concentration of zirconia in the composites increased. These results are explained as resulting from exchange of Ca2+ and ZrO2+ ions between the HA and the zirconia. The porosity of the sintered composites increased with increase in zirconia concentration in the composites, probably because of water formed by the decomposition of the HA. (C) 2006 Elsevier Ltd and Techna Group S.r.l. All rights reserved.