We describe two similar patients with a clinical diagnosis of corticobasal ganglionic degeneration (CBGD). After a period of increased action tremor, both patients developed a fixed posture in the right arm with a slow rhythmic myoclonus, which appeared to be caused by trains of highly synchronized and stimulus sensitive myoclonic discharges. Resetting of the spontaneous myoclonic discharges by peripheral and central stimulation and a jerk-locked cortical potential were demonstrated in one case. The somatosensory evoked potentials (SEPs) showed abnormal parietal curves with small N20-P25 amplitudes and without giant SEP characteristics. The latencies of the cortical event and of the late responses, and the duration and distribution of the discharges compare best with those of the cortical reflex type of myoclonus. Localized parietal cortical damage, as indicated by clinical evidence and imaging techniques, may well explain the absence of a giant SEP in these patients with CBGD.