This study aimed to determine the most suitable site for diagnosis of carpal tunnel syndrome (CTS) by examining an 8-site measurement of the median nerve’s cross-sectional area (CSA). A total of 36 wrists of 26 patients with nerve conduction study (NCS) proven CTS, along with 34 wrists of 23 controls whose age and gender were matched with the patients, were evaluated with ultrasonography. The CSAs of the median nerve at eight predetermined sites including at the sites of 3, 2, and 1 cm proximal to the wrist crease, wrist crease, as well as at the sites of 1, 2, 3. and 4 cm distal to the wrist crease were obtained. The correlation between CSA and NCS severity, and duration of clinical CTS symptoms was analyzed. Receiver operating characteristic (ROC) curves was applied to determine the optimum cut-off point and to evaluate the diagnostic sensitivity and specificity of sonographic measurements. The CSAs of the median nerves at the eight sites were significantly higher in the CTS subjects, relative to the controls. Moreover, anatomical variation of the median nerve was found in the CTS group. ROC results indicated the areas under curve (AUC) at the site of 4 cm distal to the wrist crease were the largest with 0.874 cm2, and an optimal cut-off value of 0.095 yielded a sensitivity of 88.9 % and a specificity of 76.5 %. The CSAs of “CTS-wrists” positively correlated with NCS severities and the CTS symptoms duration. Using 8-site CSAs measurement of the median nerve from inlet to outlet has positive correlations with NCS severity and duration of CTS symptoms.