Objective: To evaluate the diagnostic value of ultrasonography in patients with electrophysiologically confirmed carpal tunnel syndrome. Design: A prospective ultrasonographic study of 35 wrists with electrophysiologically confirmed carpal tunnel syndrome and of 40 normal wrists. Receiver-operating- characteristics curves for the ultrasonographic measurements of median nerve were plotted to identify the most optimal cutoff values. Results: The ultrasonographic measurements of median nerves were found to be increased significantly in patients with carpal tunnel syndrome when compared with controls, particularly in terms of cross-sectional area (P < 0.001) and the bowing of the flexor retinaculum (P < 0.01) but not in the flattening ratio (P > 0.05). According to receiver-operating-characteristics curve results, the most optimal cutoff value for the cross-sectional area of the median nerve was obtained at the level of middle carpal tunnel, which was 9.3 mm2, with a sensitivity of 80% and specificity of 77.5%. The optimal cutoff value for the bowing of the flexor retinaculum was 3.7 mm, with a sensitivity of 71.4% and specificity of 55%, No optimum cutoff value could be identified from the receiver-operating-characteristics curves for the flattening ratio of median nerve. Conclusion: Ultrasonographic examination of the median nerve seems to be a promising method in the diagnosis of carpal tunnel syndrome, evaluating the morphologic changes of the median nerve in patients with clinical signs and symptoms. Further studies with wider series are needed to confirm our preliminary results. Copyright © 2005 by Lippincott Williams & Wilkins.