A 45-year-old male presented with a history of several months of right lateral knee pain. He also complained of a creak and a movable mass at the lateral joint area during ambulation. He was a member of an amateur soccer team. An MRI was performed using a Magnetom Vision 1.5-T MR imaging unit (Siemens Medical Systems, Erlangen, Germany). The MRI showed a thickened ITB and poorly defined high signal intensity abnormalities in a compartment-like space bounded laterally by the ITB on coronal fat suppressed proton density weighted images (repetition time [TR] 1700 ms, echo time [TE] 10 ms) (Fig. 1A). The T1-weighted image (TR 400 ms, TE 20 ms) showed a decreased signal intensity lesion compared to adjacent fat, deep to the ITB (Fig. 1B). A well-demarcated, ovoid nodular lesion was also observed in the compartment-like space. The nodule showed iso-signal intensity compared to the knee muscle on a T1-weighted image, high signal intensity on T2-weighted images (TR 3200 ms, TE 100 ms), and a slightly higher signal intensity with thin rim on fat suppressed proton density weighted images (Fig. 1A-C). The initial diagnosis based on the MRI findings, clinical history, and physical examination was ITB friction syndrome. We suspected the nodular lesion was a ganglion, focal synovial thickening, focal villonodular synovitis, or focal degenerative change of invaginated extraaritcular fatty tissue. Unfortunately, we did not consider the possibility of a FTS. During an arthroscopic examination, the surgeon found an inflamed lateral synovial recess, which is typically observed in ITB friction syndrome (Fig. 1D). After a sequential arthroscopic inflamed synovial recess resection with synovial shaver, a whitish intraarticular polypoid nodule was observed, attached to the lateral joint capsule (Fig. 1E). The nodule was successfully resected and was submitted to the Department of Pathology for histologic analysis. In the resected nodule, abundant collagen fibers and scattered fibroblasts were detected. The nodule did not contain necrosis, mitotic activity, or cellular atypia (Fig. 1F, G). The nodule was diagnosed as a FTS based on the histological findings. Arthroscopic debridement of the adjacent tissues near the surface between ITB and lateral femoral condyle was also performed to relieve the symptoms of ITB friction syndrome. The resected tissues showed fibrosis, marked hemorrhage, prominent capillary proliferation, and mild chronic inflammation that was consistent with ITB friction syndrome (Fig. 1H).