Most patients with anterolateral ankle pain report some history of trauma as the precipitating event. In the majority of cases of anterolateral ankle pain with no history of trauma the cause is proliferative synovitis, especially in the area of the anteroinferior tibiofibular ligament [1,2]. Our case report is about a patient with anterolateral ankle pain and snapping, with no history of trauma, caused by an abnormal peroneal tertius muscle belly. We have found only one similar case reported in the literature . That particular case was treated with arthroscopic resection, which requires specialist arthroscopic skills. In our case we have done an open resection, which can be safely performed by most surgeons.