OBJECTIVE: CD248 (TEM1/Endosialin) is found on stromal cells and is highly expressed during malignancy and inflammation. The CD248 knockout (CD248(-/-) ) mouse has reduced inflammatory arthritis. The aim of this study was to investigate the functional effect of genetic deletion of CD248 on bone mass. METHODS: Western blotting, PCR and immunofluorescence were used to investigate CD248 expression. MicroCT and 3-point bending of tibiae from 10 week old wildtype or CD248(-/-) were used to measure bone parameters and mechanical properties. Primary osteoblasts were cultured in media containing 10 mM β-glycerophosphate and 50 μg/ml ascorbic acid to induce mineralisation and treated with PDGF-BB. Mineral apposition rate in vivo was calculated by labelling newly formed bone with calcein. RESULTS: Expression of CD248 was seen in human and mouse osteoblasts but not osteoclasts. CD248(-/-) tibiae had higher bone mass and superior mechanical properties (increased load required to cause fracture) compared to WT. CD248(-/-) primary osteoblasts had increased mineralisation in vitro and produced increased bone over 7 days in vivo and failed to decrease mineralisation or increase proliferation in response to PDGF-BB stimulation due to a defect in PDGF signal transduction. CONCLUSION: There is an unmet clinical need to address RA-associated bone loss. Genetic deletion of CD248 in the mouse results in high bone mass due to increased osteoblast-mediated bone formation, suggesting that targeting CD248 in rheumatoid arthritis may have the effect of increasing bone mass as well as the previously reported effect of reducing inflammation.