A recent study showed that carbon nanohorns (CNHs) have biocompatibility and possiblemedical uses such as in drug delivery systems. It was reported that some kinds of carbonnanomaterials such as carbon nanotubes were useful for bone formation. However, theeffect of CNHs on bone tissue has not been clarified. The purpose of this study was toevaluate the effect of CNHs on bone regeneration and their possible application for guidedbone regeneration (GBR). CNHs dispersed in ethanol were fixed on a porouspolytetrafluoroethylene membrane by vacuum filtration. Cranial defects were created inrats and covered by a membrane with/without CNHs. At two weeks, bone formation underthe membrane with CNHs had progressed more than under that without CNHs andnumerous macrophages were observed attached to CNHs. At eight weeks, there was nosignificant difference in the amount of newly formed bone between the groups and theappearance of macrophages was decreased compared with that at two weeks. Newly formedbone attached to some CNHs directly. These results suggest that macrophages induced byCNHs are related to bone regeneration. In conclusion, the present study indicatesthat CNHs are compatible with bone tissue and effective as a material for GBR.