There is an increased use of nanophase titanium dioxide (TiO(2)) in bone implants and scaffolds. However, nano-debris is generated at the bone-biomaterial interface. Therefore, TiO(2) nanoparticles (NPs) of many sizes were investigated for cytotoxic effects on murine MC3T3-E1 preosteoblasts. These TiO(2) NPs induced a time- and dose-dependent decrease in cell viability. There was a significant increase in lactate dehydrogenase (LDH) release, apoptosis and mitochondrial membrane permeability following short-term exposure of the cells to TiO(2) NPs. These NPs also increased granulocyte-macrophage colony stimulating factor (GM-CSF) and granulocyte colony-stimulating factor (G-CSF) gene expression. Compared with the 32 nm TiO(2) NPs, 5 nm TiO(2) NPs were more toxic, induced more apoptosis, increased mitochondrial membrane permeability and stimulated more GM-CSF expression at a high concentration (≥100 μg/ml). The results implied that the differential toxicity was associated with variations in size, so more attention should be given to the toxicity of small NPs for the design of future materials for implantation.