OBJECTIVE. This study was performed to elucidate the MR imaging findings and pit-falls for the diagnosis of anterolateral soft-tissue impingement in the ankle, a cause of chronic ankle pain that can be relieved by arthroscopic resection. MATERIALS AND METHODS. We retrospectively reviewed MR imaging examinations of 18 patients with arthroscopically confirmed anterolateral ankle impingement. The MR images of 18 additional subjects with symptoms that could mimic anterolateral impingement, but who had a surgically confirmed alternate diagnosis (instability, peroneal tendon injury, osteochondral defect normal arthroscopy) and no evidence of impingement at arthroscopy, served as controls. RESULTS. On the MR imaging studies, nine patients had an ankle effusion, eight of whom showed an abnormal soft-tissue structure in the anterolateral gutter, 2-15 mm in maximal diameter. No soft-tissue mass was seen in the patients without joint fluid. Four control subjects with instability had a similar soft-tissue structure in the anterolateral gutter, but in the control subjects the finding represented a portion of the torn anterior talofibular ligament. CONCLUSION. Anterolateral soft-tissue impingement of the ankle can be suggested by MR imaging when fluid in the lateral gutter outlines an abnormal soft-tissue structure separate from the anterior talofibular ligament.