OBJECTIVE: To investigate the effects of the hypoxia inducible factor-1 (HIF-1) activation-mimicking agent cobalt chloride (CoCl2) on the osteogenic differentiation of human mesenchymal stem cells (hMSCs) and elucidate the underlying molecular mechanisms. STUDY DESIGN: The dose and exposure periods for CoCl2 in hMSCs were optimized by cell viability assays. After confirmation of CoCl2-induced HIF-1alpha and vascular endothelial growth factor expression in these cells by RT-PCR, the effects of temporary preconditioning with CoCl2 on hMSC osteogenic differentiation were evaluated by RT-PCR analysis of osteogenic gene expression, an alkaline phosphatase (ALP) activity assay and by alizarin red S staining. RESULTS: Variable CoCl2 dosages (up to 500 microM) and exposure times (up to 7 days) on hMSC had little effect on hMSC survival. After CoCl2 treatment of hMSCs at 100 microM for 24 or 48 hours, followed by culture in osteogenic differentiating media, several osteogenic markers such as Runx-2, osteocalcin and osteopontin, bone sialoprotein mRNA expression level were found to be up-regulated. Moreover, ALP activity was increased in these treated cells in which an accelerated osteogenic capacity was also verified by alizarin red S staining. CONCLUSIONS: The osteogenic differentiation potential of hMSCs could be preserved and even enhanced by CoCl2 treatment.