Inhibition of heat-shock protein 27 expression eliminates drug resistance of osteosarcoma to zoledronic acid. Although zoledronic acid (ZOL) has been reported to show anti-tumour effects on various malignant tumours, few studies have reported the molecular mechanisms of resistance to ZOL.A drug-resistant cell line was developed by repeatedly treating human osteosarcoma cell line HOS with ZOL. Expression status of drug resistance-related molecules including ATP-binding cassette (ABC) and heat-shock protein 27 (HSP27) was confirmed in order to analyse molecular mechanisms of the drug resistance.Repeated treatment with ZOL induced drug resistance with down-regulation of apoptosis in the resistant cell line. Although ABC expression was down-regulated, up-regulation of HSP27 in the resistant cell line was confirmed. The resistance was overcome by HSP27 silencing.Resistance to ZOL in osteosarcoma cells may be induced by a molecular mechanism different from conventional efflux pump-based resistance. Up-regulation of HSP27 expression may play a role in the development of ZOL resistance in HOS cells.