Remifentanil is commonly used in operating rooms and intensive care units for the purpose of anesthesia and sedation or analgesia. Although remifentanil may significantly affect the bone regeneration process in patients, there have been few studies to date on the effects of remifentanil on bone physiology. The purpose of this study was to investigate the effects of remifentanil on osteoclast differentiation and bone resorption. Bone marrow-derived macrophages (BMMs) were cultured for 4 days in remifentanil concentrations ranging from 0 to 100 ng/ml, macrophage colony-stimulating factor (M-CSF) alone, or in osteoclastogenic medium to induce the production of mature osteoclasts. To determine the degree of osteoclast maturity, tartrate-resistant acid phosphatase (TRAP) staining was performed. RT-PCR and western blotting analyses were used to determine the effect of remifentanil on the signaling pathways involved in osteoclast differentiation and maturation. Bone resorption and migration of BMMs were analyzed to determine the osteoclastic activity. Remifentanil reduced the number and size of osteoclasts and the formation of TRAP-positive multinuclear osteoclasts in a dose-dependent manner. Expression of c-Fos and NFATC1 was most strongly decreased in the presence of RANKL and remifentanil, and the activity of ERK was also inhibited by remifentanil. In the bone resorption assay, remifentanil reduced bone resorption and did not significantly affect cell migration. This study shows that remifentanil inhibits the differentiation and maturation of osteoclasts and reduces bone resorption.