BACKGROUND: Although nerve conduction study (NCS) is the method most frequently used to confirm clinical diagnosis of carpal tunnel syndrome (CTS), ultrasonographic (US) measurement can give additional information to confirm the diagnosis and also exclude other conditions of nearby soft tissues. However, whether or not the degree of swelling of median nerve (MN) reflects clinical severity has not been proven before. This study is aimed to investigate the further clinical usefulness of US in assessing CTS severity. METHODS: One hundred and twenty-four patients (248 hands) with electrophysiologically confirmed CTS were evaluated. Clinical severity was examined by Historic and Objective (Hi-Ob) scale. Padua scale was used for the severity of electrophysiological impairment. For US study, cross-sectional area (CSA) of the median nerve was measured at the proximal inlet of the carpal tunnel and graded. RESULTS: Ninety-four patients were female (75%) and median disease duration was 19 weeks. There was a good correlation between electrophysiological impairment and CSA of median nerve (correlation coefficient=0.442, p<0.001), and CSA was graded as US severity scale by electrophysiological severity of patients. A statistically significant correlation was found among US severity scale of the MN at wrist, clinical severity scale (correlation coefficient=0.397, p<0.001), and electrophysiological severity scale (correlation coefficient=0.371, p<0.001). CONCLUSIONS: This observation suggests MN swelling in CTS may reflect in itself the degree of nerve damage as expressed by the clinical picture. US measurement could also give additional information about severity of MN involvement above the diagnosis of CTS.