OBJECTIVE: To find out the incidence of reduced median conduction velocity of forearm (MNCV-F) in carpal tunnel syndrome (CTS) and to compare clinical and electrophysiologic characteristics of CTS with reduced MNCV-F and to observe the changes of reduced MNCV-F after carpal tunnel release. METHOD: One hundred and fifty nine hands with CTS are divided into two groups; MNCV-F of 50 m/sec and above as group I and that of below 50 m/sec as group II. For the electrophysiologic comparison, median sensorimotor distal latency, peak-to-peak amplitudes and abnormal spontaneous activity of abductor pollicis brevis were observed and for clinical comparison, sensorimotor symptoms, Phalen and Tinel sign were observed. Twenty four hands which had successful carpal tunnel release were examined for the changes of MNCV-F. RESULTS: The hands with reduced MNCV-F were 29 among 159 hands. Sensorimotor distal latency were significantly prolonged and sensorimotor amplitudes also significantly reduced in group II. Sensory change and Phalen signs were more frequently observed in group II. MNCV-F in group I had not changed after carpal tunnel release, but MNCV-F in group II was improved significantly. The changes MNCV-F in group II were much delayed than the improvement of parameters of distal conduction studies. CONCLUSION: The incidence of reduced MNCV-F in CTS was 18.24%. Patients with reduced MNCV-F had more severe CTS both electrophysiologically and clinically. Reduced MNCV-F had improved significantly, but there was significant time gap between the electrophysiologic improvements of distal and proximal portions of nerve. This findings may suggest that retrograde degeneration may play a partial role in reduced forearm motor nerve conduction velocity of the median nerve in CTS.