There are certain methods to treat craniomaxillofacial bone defects. If one does not want to use autogenous bone, hydroxyapatite (HAP) might be an alternative bone substitute. Although it has the advantage of no donor area morbidity, a sinking phenomenon has been found to occur; its degree differs from site to site. In the present study, we investigated the effect of the site of the onlay graft and differences in its porosity on the sinking phenomenon. The experiment was performed by inserting HAP blocks having 30% and 50% porosity under the periosteum at three sites : the frontal bone, the zygoma, and the mandible, of mature male New Zealand White (NZW) rabbits. Three NZW rabbits were allocated to a sham group, and 16 rabbits were sacrificed 8 weeks after grafting. Villanueva's bone stain was used to histologically examine the interface between the HAP and the recipient bone for autogenous bone substitution and migration, identified by the presence of a mixture of osteoblasts and osteoclasts. The order of severity of the sinking phenomenon according to site was: mandible >zygomatic bone >frontal bone, and according to porosity: 50%>30%; and clear differences from the sham groups were shown. Based on the above findings, the results of the experiment showed that selecting porosity after considering the strength of the implant and the site to be grafted is an important factor when applying HAP clinically. When comparing the present experiment's results with Yamada et al's study results, it can be concluded that the degree of sinking is lesser in mature rabbits than immature rabbits. © 2001, The Showa University Society. All rights reserved.