OBJECTIVE: To describe the sonographic appearance of a poorly recognized cause of low thoracic or upper abdominal pain. METHODS: Three sonographic descriptions of slipping rib syndrome are presented. RESULTS: The 3 patients had abnormal mobility of a cartilaginous rib, which could slip over an adjacent rib during abdominal muscle contraction. CONCLUSIONS: Slipping rib syndrome should be considered in patients with histories of upper abdominal or low thoracic pain of unknown origin. We suggest that high-resolution sonography of the costal margin should be added to abdominal sonography in cases of nonspecific abdominal pain.