Motor-evoked potentials reflect spinal cord function in post-traumatic syringomyelia. The purpose of this study was to examine electrophysiologic abnormalities, including motor-evoked potentials, in a patient with post-traumatic syringomyelia before and after syringopleural shunting. A patient with C5 quadriplegia presented with pain, ascending sensory loss, and new weakness in the left upper extremity two yr after spinal cord injury. MRI revealed a syrinx extending from C2 to T12. We measured central motor conduction times (CMCTs) to the biceps, median F-wave latencies, needle electromyography and motor nerve conduction studies. Six days before surgery, CMCTs were 9.0 ms on the left and 7.8 ms on the right (normal less than 8.0), median F-waves were absent on the left and needle EMG revealed evidence of denervation in the left biceps. Fifteen days after syringopleural shunting at the T7 level, CMCTs had dropped to 6.9 ms on the left and 4.6 ms on the right; the left median F-wave reappeared with a normal latency. Repeat MRI revealed the syrinx to be smaller in diameter. These results suggest that CMCTs measured from magnetic stimulation of the motor cortex may be useful in the diagnosis of post-traumatic syringomyelia, as well as for following such patients postoperatively. 