BACKGROUND: The objectives of our study were to assess the effects of oxidative stress on the proliferation, differentiation and apoptosis of human bone marrow stromal cell (BMSC)-derived osteoblasts and to explore pathways by which osteoblast cell apoptosis was induced. METHODS: Mononuclear cells including BMSCs were cultured to osteoblastic lineage. Different doses of hydrogen peroxide (H2O2) were added to the culture media. The colony forming units-fibroblastic (CFU-Fs) were stained with crystal violet and alkaline phosphatase (ALP). The MTT assay was done to see the effect of H2O2 on cell viability. The effect of H2O2 on osteocalcin gene expression was determined by RT-PCR. The matrix calcification measurement was performed. FACS analysis was performed to determine the osteoblasts apoptosis. Caspase-3, -8 and 9 activity assay and cytochrome c release were measured. RESULTS: The size and number of ALP (+) CFU-Fs were also decreased by H2O2 treatment. When compared with the control group, H2O2 significantly decreased the total number of cells of each culture well during MTT assay. H2O2 significantly diminished expression of osteocalcin mRNA. N-acetylcystein (NAC) blocked the diminution of cell viability and the inhibition of osteocalcin mRNA expression by H2O2. H2O2 reduced matrix calcification. FACS analysis revealed H2O2 increased percentage of apoptotic cells. Addition of H2O2 resulted in the increase of caspase-9 and -3 activity but not caspase-8, and release of cytochrome c to the cytosol. CONCLUSION: These data suggest that, in primary human BMSCs, oxidative stress inhibits proliferation of stromal cells and inhibits the differentiation to osteoblastic lineage. In addition, oxidative stress induces apoptosis of human BMSC-derived osteoblasts and this may be mediated by mitochondrial pathway of apoptotic signal.