Abstract: Carpal Tunnel Syndrome (CTS) is a common problem that is caused by pressure on a nerve in the wrist and is associated with various factors. We compared Cross-Sectional Area (CSA) of the inlet and outlet in CTS patients and healthy controls and define the best cut off point of CSA and assess the utility of US in the diagnosis of CTS patients as well. This cross-sectional, age-group-matched case-control study was performed on 39 patients aged 42-65 years with electro-physiologically confirmed idiopathic CTS and 35 healthy controls from Golestan Hospital, Ahvaz, Iran from Jan 2013 to Feb 2014. Neurological and electro-diagnostic tests for patients were performed by an expert neurologist as the gold standard diagnostic test for CTS cases. Hundred wrists were evaluated. Electromyography (EMG) showed that 16 (32%) cases were mild, 17 (34%) were moderate and 17 (34%) were severe. The results showed that the CSA of carpal tunnel inlet, carpal tunnel outlet and AP diameter were significantly different between the two groups (p<0.001, p = 0.009, respectively). The sensitivity and specificity of carpal tunnel inlet were 92 and 96%, for carpal tunnel outlet were 92 and 92%, for AP diameter were 64 and 58%. longitudinal evaluations are necessary to better know the utility of ultrasound for diagnosing CTS. The accuracy of diagnosis increases in parallel with the increase in the stages and severity of CTS. Therefore, patients with severe CTS would not need the more uncomfortable, costly and invasive techniques.