This study compared the osteoconductive capability of deproteinized bone particles of two different sizes (300-500 and 850-1000 microm) in rabbits undergoing maxillary sinus lift. Histologically, deproteinized bone particles of both sizes induced osteoconduction 1 week after implantation. Bone initially formed at the sinus wall and proliferated into the center of the augmented sinus cavity. In the small-particle group, newly formed bone showed many interconnections and appeared in most areas of the cavity 8 weeks after implantation. In the large-particle group, newly formed bone showed limited intercommunications, and the center of the sinus cavity contained fibrous connective tissue with no evidence of ossification 8 weeks after implantation. Histomorphometric analysis revealed a significantly higher density of newly formed bone in the small-particle group than in the large-particle group both 4 and 8 weeks after implantation. The total newly formed bone-particle contact length was also significantly higher in the small-particle group. The total surface length of the small particles was larger than that of the large particles, but the ratio of the newly formed bone-particle contact length to the total particle surface length did not differ significantly between the groups at any time. The interparticular spaces of the small particles were larger than those of the large particles. The bone area ratio in the interparticular spaces of the small particles was significantly higher than that of the large particles both 4 and 8 weeks after implantation. We conclude that graft bone particle size and interparticular space are important determinants of osteoconduction.