Microtubules have a critical role in cell division, and consequently various microtubule inhibitors have been developed as anticancer drugs. In this study, we assess mebendazole (MZ), a microtubule-disrupting anthelmintic that exhibits a potent antitumor property both in vitro and in vivo. Treatment of lung cancer cell lines with MZ caused mitotic arrest, followed by apoptotic cell death with the feature of caspase activation and cytochrome c release. MZ induces abnormal spindle formation in mitotic cancer cells and enhances the depolymerization of tubulin, but the efficacy of depolymerization by MZ is lower than that by nocodazole. Oral administration of MZ in mice elicited a strong antitumor effect in a s.c. model and reduced lung colonies in experimentally induced lung metastasis without any toxicity when compared with paclitaxel-treated mice. We speculate that tumor cells may be defective in one mitotic checkpoint function and sensitive to the spindle inhibitor MZ. Abnormal spindle formation may be the key factor determining whether a cell undergoes apoptosis, whereas strong microtubule inhibitors elicit toxicity even in normal cells.