Aim: The purpose of this study was to evaluate the effects of the addition of nano-hydroxyapatite to a collagen membrane-carrier of recombinant human bone morphogenetic protein-2 (rhBMP-2) on hard tissue formation and dentin resorption on dentin surfaces in vivo. Materials and methods: Nano-hydroxyapatite collagen composite (nHAC) membranes or collagen (C) membranes were each immersed in either 100 or 400 mu g/ml rhBMP-2 and placed on dentin chips that were implanted into rat thigh muscle. The implants were analyzed at 2 or 4 weeks after surgery by histological observation and histomorphometric analysis. Results: The percentage of the hard tissue formed by each nHAC group was significantly higher than that formed by any of the C groups, except for that formed by the group loaded with 400 mu g/ml rhBMP-2 at 4 weeks after implantation. No significant differences were observed in the percentage of dentin resorption between the nHAC groups and C groups at any stage or at any rhBMP-2 concentration. Conclusion: These findings showed that addition of nano-hydroxyapatite to a collagen membrane accelerated the formation of hard tissue induced by a low dose of rhBMP-2 on dentin surfaces at an early stage after implantation into rat thigh muscle, without increasing dentin resorption. (c) 2012 Elsevier B.V. All rights reserved.