Osteosarcoma is one of the most common primary malignant tumors of the bone in children and adolescents. Some patients continue to have a poor prognosis, as they have metastatic disease and frequent occurrence of drug resistance. Zoledronate is a nitrogen-containing bisphosphonate that has been used for the treatment of hypercalcemia and bone metastasis, because it induces apoptosis in osteoclasts and tumor cells by inhibiting the isoprenylation of intracellular small G proteins. Besides inhibiting isoprenylation, little is known about the manner by which bisphosphonates inhibit cellular proliferation and induce apoptosis. This prompted us to investigate the inhibitory effects of zoledronate in human osteosarcoma cell lines, HOS and MG63. HOS cells accumulated in S phase around 6 h after treatment with 10 microM zoledronate, followed by apoptosis. When HOS cells were treated with zoledronate, ATM kinase and its substrate, check-point kinase (Chk)1, were phosphorylated. Zoledronate also induced phosphorylation of cdc25a (Thr506) in HOS cells, which is a substrate of Chk1, and its phosphorylation is known to be critical for S phase arrest. Following treatment with zoledronate, phosphorylated histone H2AX (gamma-H2AX) displayed patterns of nuclear foci in HOS cells. As gamma-H2AX accumulates at dsDNA breaks, these results demonstrate that zoledronate induced DNA damage and S phase arrest, accompanied by activation of the ATM/Chk1/cdc25 pathway in a human osteosarcoma cell line.