OBJECTIVE: The purpose of this study was to find out what was the diagnostic usefulness of ultrasonography in carpal tunnel syndrome METHOD: Subjects included 30 hands of 25 carpal tunnel syndrome patients and 20 hands of 17 healthy adults with no electrophysiologic evidence of carpal tunnel syndrome. All subjects were examined with real-time ultrasonography. The ultrasonography was evaluated quantitatively with regard to the cross-sectional area and the flattening ratio of the median nerve and the palmar displacement of the flexor retinaculum. The analysis of difference between normal control group and carpal tunnel syndrome group was done by independent t-test. RESULTS: The carpal tunnel syndrome patients group had significantly increased cross-sectional area of the median nerve at distal radio-ulnar joint, pisiform and hamate. The carpal tunnel syndrome patients group had significantly increased flattening ratio of median nerve at hamate. The carpal tunnel syndrome patients group had significantly increased palmar displacement of the flexor retinaculum. CONCLUSION: Based on the result of this study, the ultrasonography is useful in the diagnosis of the carpal tunnel syndrome.