OBJECTIVE: We investigated the useful electrodiagnostic indicators and the extent of retrograde degeneration (RD) in carpal tunnel syndrome (CTS). METHOD: We measured median mixed nerve action potentials (MMNAP) in 26 CTS and 37 normal hands by recording at the elbow and stimulating 0, 3, 6 and 9 cm proximal to the distal wrist crease. The 3 MMNAP parameters such as amplitude, latency and conduction velocity were compared between CTS and control group. The most useful indicator was compared between subgroups of CTS (mild and severe) and control group. RESULTS: The amplitudes of all MMNAPs in CTS group, except stimulating 9 cm proximal to the wrist (MA9), were significantly smaller than those in control group (p<0.05). MA9 in severe CTS subgroup, not mild subgroup, was significantly smaller than that in control group (p<0.05). CTS and control group were not significantly different in the MMNAP latencies, except stimulating 9 cm proximal to the wrist (p<0.05), and in the MMNAP conduction velocity, except stimulating in 0 cm to 3 cm segment proximal to the wrist. CONCLUSION: The amplitude of MMNAP in forearm can be the most useful indicator of RD in CTS, and the conduction velocity, a less useful indicator. We believe that RD progresses as the severity of CTS increases, and dose beyond 9 cm proximal to the distal wrist crease.