PURPOSE: To describe the ultrasonographic (US) findings of carpal tunnel syndrome (CTS) and to evaluate the diagnostic value of US in CTS. MATERIALS AND METHODS: Forty-four wrists of 26 patients aged 35 to 67 (mean, 52) years with CTS who were electrophysiologically diagnosed, and 30 wrists of 15 normal control subjects aged 33-62(mean, 48 years) were studied using US with a 7.5MHz linear transducer. Axial images of these wrists in the neutral position were obtained at the level of the distal radius, pisiform, and hook of hamate. The following measurements were taken: at each level, cross sectional area (CSA) and flattening ratio (FR) of the median nerve; at the hamate level, bowing of the flexor retinaculum (palmar displacement: PD); during passive flexion and extension of the index finger, transverse sliding of the median nerve. RESULTS: CSA at each level was significantly higher in patients than in controls (p=0.0001): 9.29 +/-2. 63 mm 2(mean +/-S.D.) vs 5.45 +/-1.98 mm 2 at the distal raidus; 10.68 +/-3.38 mm 2 vs 6.55 +/-2.01 mm 2 at the pisiform; 1 0 . 8 8 +/-2.78 mm2, vs 6.34 +/-2.00 mm2 at the hamate. FR was significantly higher in patients(2.37+/-0.56) than in controls (2.06 +/-0.36) only at the level of the hamate (p=0.0064). In additi on, PD of the flexor retinaculum was also significantly higher in patients (3.44 +/-0.90 mm) than in controls (2.20 +/-0.55mm) (p=0.0001). The sliding distance of median nerve during passive flexion and extension of the index finger was, however, significantly lower in patients (0.98 +/-1.03 mm) than in controls (1.65 +/-1.22 mm) (p=0.0180). CONCLUSION: For the diagnosis of CTS, US proved useful. Significant ultrasonographic findings in CTS were swelling of the median nerve, increased flattening ratio of the median nerve at the distal carpal tunnel, increased bowing of the flexor retinaculum, and decreased mobility of the median nerve during motion of the index finger.