Since the turn of the century there has been a large scale increase into research on epigenetics. One such way of regulating gene expression without altering base pair sequencing is through the acetylation and de-acetylation of histone proteins. Over the past decade a number of histone deacetlyation inhibitors (HDACi) have been used successfully to teat cancer. Recently more specific HDACi have been developed and these compounds have been shown to regulate the activities of a variety of cells at concentration much lower (10-100 fold) than their chemotherapeutic effects. Our recent publications have demonstrated that HDACi that inhibit specific HDAC enzymes can have a profound effect on osteoclast formation and activity in vitro(1) and in vivo(2). In diseases such as periodontitis, rheumatoid arthritis, and peri-prosthetic osteolysis an imbalance between bone resorption and bone formation occurs possibly through the effects of inflammatory cytokines such as tumour necrosis factor-α (TNFα). Based on our studies of the HDAC enzymes expressed in bone pathologies(3), we investigated how specific HDACi targeting HDAC 1 or 5 can regulate human osteoblast differentiation and bone formation. Human osteoblastic cells from 4 donors were isolated from bone fragments collected at time of surgery. We found that the inhibition of HDAC 5 significantly enhanced RUNX2 expression (p<0.05). Similar increases were seen in other markers of bone formation, OCN, OPN, and COL1A1. In addition, HDAC5 inhibition induced substantial increases in mineralized deposits and alkaline phosphatase activity (p<0.05). Similar results were observed in the presence and absence of TNFα. Overall our findings show that HDAC 5 has an important role during osteoblast differentiation, and HDAC 5 inhibition may enhance bone formation. These results show that epigenetic regulation of bone metabolism may be an effective way of treating bone loss disease.