Abstract The slipping rib syndrome is characterized by burning pain in the lower thorax and upper abdomen, often disabling, caused by hypermobility of the costal cartilage with entrapment of the intercostal nerve. The syndrome is often underdiagnosed. The diagnosis is clinical and the definitive treatment is surgical, with an excellent result for pain relief. Based on the observation of 4 cases undergoing rib resection for SRS, we noticed a new possible sign of the disease. Our patients showed less thickness of the ipsilateral rectus abdominis muscle on ultrasound of the abdomen. The aim of this study is to demonstrate this sign in the diagnosis of SRS, to make this disease more recognizable and treatable. The slipping rib syndrome (SRS) is characterized by intense pain in the lower rib arch radiating to the side and the upper abdomen, or by chest pain.