Increasing bone formation at endosseous titanium implants may be achieved by modification of topographically enhanced surfaces. The aim of this study was to determine the effect of fluoride ion modification of TiO2 grit-blasted, c.p. titanium implants on osteoblastic differentiation and interfacial bone formation by parallel in vitro and in vivo investigations. Human mesenchymal stem cells (Osiris Therapeutics, Inc.) were cultured on TiO2 grit-blasted c.p.titanium disks with and without fluoride ion modification. Cell adhesion, proliferation, and osteoblastic gene expression was measured by scanning electron microscopy, tritiated-thymidine uptake into insoluble DNA, and reverse transcription polymerase chain reaction detection of mRNAs encoding collagen 1, osteopontin, bone sialoprotein, osteocalcin and BMP-2. After 24 h, there were no differences in cell adhesion among the surfaces tested. Fluoride-treated surfaces supported greater proliferation and increased bone sialoprotein and BMP-2 expression. Additionally, 12 TiO2 grit-blasted and 12 fluoride ion modified implants were placed randomly into medial and distal osteotomies prepared in the tibia of 300 g Sprague Dawley rats. After 21 days, the tibiae were harvested and 100 microm ground sections were examined by backscatter scanning electron microscopy. The bone-to-implant contact formed at TiO2 grit-blasted and fluoride-treated versus TiO2 grit-blasted surfaces was 55.45% versus 34.21% (p<0.027), respectively. Fluoride ion modification of the TiO2 grit-blasted surface enhanced osteoblastic differentiation in vitro and interfacial bone formation in vivo. This parallel in vitro and in vivo investigation demonstrates that fluoride ion modification enhanced osteoblastic differentiation and interfacial bone formation. The mechanism(s) by which fluoride ion modification of c.p.titanium enhanced osteoblastic differentiation and osseointegration merit careful investigation.