A case of a patient with snapping pes syndrome following a blunt trauma to the knee is presented. Snapping pes syndrome is a painful catching experienced at the posteromedial corner of the knee from pes anserinus when moving from flexion to extension. Since the accident ten years ago the patient experienced a painful snapping phenomenon on the inner side of the knee. MRI and arthroscopies found no explanation. Ultrasound revealed that the semitendinosus snapped over a bony fragment during extension of the knee. Removal of the fragment resulted in remission.