Scalp topography of the giant SEP and the pre-myoclonus spike demonstrated by jerk-locked back averaging was studied by using a computer-assisted evoked potential mapping technique in 5 patients with cortical reflex myoclonus. The initial positive peak of giant SEP was localized to the postcentral region contralateral to the stimulus and was associated with a negative potential field of the same latency at the frontal region in all cases. The main positive peak of pre-myoclonus spike was localized to the postcentral contralateral to the myoclonus in 4 cases, and maximal at the midline postcentral region extending contralaterally with respect to the myoclonus in 1 case. The postcentral positive peak was associated with a frontal negativity in 2 of the 5 cases. In those 2 cases, the main components of giant SEP and the pre-myoclonus spike showed a similar scalp distribution with respect to the hand which was stimulated or myoclonic jerks were recorded from, although the latter was much smaller and less sharp than the former. These findings support our previous hypothesis that those 2 activities might be generated, at least in part, by common physiological mechanisms. In 3 other cases, however, the postcentral positive peak of the pre-myoclonus spike was not associated with a frontal negativity.