OBJECTIVE: To evaluate the influence of the length of the median nerve in the diagnosis of carpal tunnel syndrome (CTS). METHOD: Inching test of median sensory nerve using Ten Electrodes was performed in 114 hands of 68 CTS patients and 68 hands of 34 controls. The abnormal cut-off values of 1-cm, 2-cm, 3-cm, 4-cm, 5-cm, 6-cm, and 7-cm segment studies were calculated as the maximal conduction delay per centimeter (maximal CD/cm)+SD x 2 for each segment in controls. Based on such values, the sensitivity of each segment study in the CTS group and the specificity in controls were obtained. RESULTS: In controls, mean CD/cm was 0.20+/-0.08 ms, and maximal CD/cm was 0.26+/-0.08 ms in the segment between 2 and 3 cm distal to distal wrist crease. With the abnormal cut-off value set at 0.26 ms, the sensitivity of the 1-cm segment study was 98.5% in CTS. The sensitivities of the 2-cm through 7-cm segment studies were 93.9%, 93.9%, 92.4%, 92.4%, 92.4%, and 90.9%, respectively. CONCLUSION: These suggest that studies of longer nerve segments tend to lower the sensitivity of the test because the inclusion of the unaffected segments in calculation attenuates the effect of conduction delay at the site of the lesion.