To supplement existing knowledge regarding the pathophysiology of the opsoclonus-myoclonus syndrome, electrophysiological findings are reported in three patients with idiopathic opsoclonus–myoclonus. Surface electromyography (EMG) revealed <100-ms synchronous discharges correlating with the clinical myoclonus. Short duration EMG discharges, with no back-averaged cortical correlate, normal gross electroencephalogram, and no exaggerated responses with either evoked potential testing or long latency EMG responses were observed. The clinical and electrophysiological findings we describe are consistent with a brainstem origin of the myoclonus in this syndrome, with concurrent abnormalities in cerebellar circuits, similar to those described for paraneoplastic cases. The constellation of these electrophysiological findings differentiates the myoclonus in these patients from other clinical myoclonic entities.