PURPOSE: The aim of this study is to evaluate morphological changes of the medial nerve in patients with carpal tunnel syndrome (CTS) before and after endoscopic release of the transverse carpal ligament, and to correlate the ultrasonography (US) findings with the use of high resolution US and the surgical outcome for the median nerve. MATERIALS AND METHODS: Thirty patients with CTS confirmed by a clinical and electrophysiological study underwent high resolution US. The US instrumentation was equipped with a high frequency linear transducer to measure the cross sectional area, flattening ratio and swelling ratio of the medial nerve at the distal radioulnar joint, proximal and distal carpal tunnel before and three months after surgery. RESULTS: The cross sectional area (CSA) of the median nerve at the distal radioulnar level showed a decrease from 0.13 +/- 0.03 cm2 before surgery to 0.11 +/- 0.03 cm2 after surgery, and the CSA of the proximal carpal tunnel showed a decrease from 0.17 +/- 0.07 cm2 to 0.14 +/- 0.05 cm2; these differences were statistically significant. There was no statistically significant correlation between the morphological change and symptom improvement. CONCLUSION: This study confirmed a decreasing CSA of the medial nerve at the distal radioulnar and proximal carpal tunnel in a postoperative patient with CTS, as determined by the use of high resolution US. No association was found between a change in the CSA of the median nerve and symptom improvement. A further study based on multiple measurements of the median nerve with a longer period is necessary to establish the association between a change in the CSA of the median nerve and symptom improvement.