The functional coordination of the OPG/RANKL/RANK system seems to contribute not only to alveolar remodeling, but also to resorption during orthodontic tooth movement and physiological root resorption. Interleukin (IL)-17 is a member of a novel family of proinflammatory cytokines produced almost exclusively by a newly recognized subclass of activated T cells called "Th17" cells. The present study was designed to identify the expression of IL-17/RANKL in the event of root resorption provoked by excessive orthodontic force and also to explore the biological mechanism of the IL-17/RANKL system in regulating odontoclastogenesis in root cementum. Thirty male Wistar rats were subjected to orthodontic force of 100 g to induce a mesially tipping movement of the left upper first molars. The contralateral molars served as intraindividual controls. The rats were sacrificed after 3, 7, or 10 days. Immunohistochemical assays were conducted to localize the expression of IL-17/RANKL proteins and histomorphological examination was undertaken to identify cellular response in cementum, periodontal ligament, and alveolar bone during root resorption. Root resorption in cementum occurred at the sites corresponding to compression of the periodontal ligament, indicated by increased numbers of multinucleated osteoclasts and odontoclasts. Immunoreaction against RANKL became stronger in osteoblasts, osteoclasts, odontoclasts, and fibroblasts of the periodontal ligament from Day 3 onwards. The number of positively immunoreactive cells against IL-17 also increased significantly over Days 3, 7, and 10 (p<0.01). The IL-17/RANKL system is responsible for transducing the heavy mechanical loading signals into an odontoclastic episode in root resorption.