The aim of this studywas to investigate how interleukin (IL)-17 and the receptor activator of nuclear factor kappaB (RANK) ligand (RANKL) contribute to root resorption during orthodontic tooth movement. The first objective was to determine the levels of interleukin-17 (IL-17), soluble RANKL (sRANKL) and osteoprotegerin (OPG) in gingival crevicular fluid (GCF) obtained from a patient with severe root resorption after orthodontic treatment. A second objective was to investigate the effects of IL-17 on the release of sRANKL and OPG from human periodontal ligament (hPDL) cells. Subjects with severe root resorption (>1/3 of original root length) were identified. Control group subjects with no loss of root structure while undergoing orthodontic treatment were also identified. GCF was collected non-invasivelyfrom the mesial and distal sides of each of the four upper incisors using filter paper strips. Eluted GCF was used for analysis by enzyme-linked immunosorbent assay (ELISA). Furthermore, this in vitro studywas performed to examine the effects of IL-17 on RANKL and OPG release from human PDL cells using ELISA. Concentrations of IL-17, sRANKL, sRANKL/OPG ratio in GCF were significantlyhigher in subjects with severe root resorption than in those with no resorption. In the in vitro study,IL-17 increased the release of sRANKL, and decreased that of OPG from hPDL cells in a time- and dose-dependent manner. These results suggest that IL-17 stimulates orthodonticallyinduced inflammatoryroot resorption (OIIRR) via production of RANKL in PDL.