Tumor-induced osteolysis or lytic bone disease is mediated by osteoclast activation. Osteoclasts can be activated directly by products produced by tumors or indirectly through other nonmalignant cells. By reducing osteoclastic activity, bisphosphonates inhibit bone resorption. Since these agents were shown effective in treating other diseases associated with increased bone resorption, including cancer-related hypercalcemia and Paget's disease of bone, studies have been initiated to explore the use of bisphosphonates in patients with osteolytic bone metastases. Recent large randomized double-blind studies show the efficacy of these agents in reducing skeletal complications in patients with bone metastases from both breast cancer and multiple myeloma.