['To detect the mechanism of alendronate during inhibition of bone resorption by inducing apoptosis of osteoclasts and its relationship with the Fas gene.', 'The osteoclast-like cells (OCLs) from a giant cell tumor in vitro were used. After treatment with alendronate for 48 hours, the OCLs were identified by fluorescence microscope, transmission electron microscope, in situ end labeling (TUNEL), HE staining, in situ hybridization with Dig-Fas Probe, and immunohistochemistry with anti-Fas antibodies.', 'The detachment rate of OCLs was 76% after exposure to alendronate, while the control cell detachment rate was only 3%. The nonadherent OCLs showed the features of cell contraction, pyknosis, chromatin condensation connecting with nuclear membrane to form circle or crescent bodies, and occasional nuclear fragmentation. The detached OCLs were positive by in situ end labeling, and the control group was negative. Nonadherent OCLs were ultrastructurally consistent with apoptosis (shrunken cells with pyknotic nuclei, chromatin condensation connecting with nuclear membrane, mitochondria recruitment with intact structure, and loosening of endoplasmic reticulum). The results by in situ hybridization and immunohistochemistry indicated that the Fas gene expression of nonadherent OCLs were positive, but the controls were negative.'] Is apoptosis of osteoclast-like cells induced by alendronate related to Fas gene expression?