Motor conduction velocity (CV) measurements often show conduction velocity slowing in the forearm segment of the median nerve in carpal tunnel syndrome (CTS). This is thought to be caused by either retrograde demyelination in the forearm or conduction block of the fastest fibers within the carpal tunnel. Direct measurement of the forearm segment of the median nerve should distinguish these two possibilities. Standard motor conduction studies and mixed nerve action potential (MNAP) recordings were performed on the forearm segment of the median nerve in patients (n = 32 limbs, aged 24 to 76) and controls (n = 15 limbs, aged 33 to 76). CVMot and CVMNAP were compared between patients and controls. The CVMot was significantly slower in patients with CTS than controls (patients 48 ± 5 msec, controls 54 ± 5 m/s, P = 0.001), but the CVMNAP showed no difference (patients 59 ± 5 m/s, controls 60 ± 5 m/s, P = 0.5) in the two groups. Additionally, the difference between CVMNAP and CVMot in a given patient was significantly larger in the patient group than in the controls (patients 11 ± 4 m/s, controls 6 ± 2 m/s, P < 0.0001). These results strongly support the hypothesis that the slowing seen in the forearm is caused by conduction block of the fastest conducting fibers within the carpal tunnel and is not caused by retrograde demyelination.