We examined the relationship between decreases in the amplitude of the compound muscle action potential (CMAP), caused by ischemic and compressive insults to the spinal cord, and postoperative motor deficits. Results were compared with those for other evoked potentials commonly used for multimodal monitoring of the spinal cord. CMAP was more sensitive than the other evoked potentials employed to ischemic and compressive insults to the spinal cord, although the disappearance of CMAP did not always result in a residual motor deficit. A decrease of more than 50% in the amplitude of the motor-evoked potential (MEP) from the spinal cord correlated well with the postoperative motor deficit. CMAP is a sensitive tool for the early detection of spinal cord impairment caused by ischemic or compressive insults to the spinal cord. The time after the disappearance of the CMAP amplitude was important for predicting postoperative motor deficit, but it is also necessary to employ CMAP concomitantly with other conductive potentials in spinal cord monitoring.