In the few reports where electrophysiologic techniques have been used to characterise stimulus sensitive myoclonus in the setting of a parkinsonian syndrome, the origin of the myoclonus has usually been found to be cortical. We describe a patient with parkinsonism unresponsive to levodopa who had myoclonus that was both spontaneous and induced by somatosensory stimuli. In addition, autonomic symptoms and a marked sleep disturbance were present early in his illness. Results of electrophysiologic investigations including electromyography (EMG) studies, routine electroencephalography (EEG) recording, jerked locked back-averaging of EEG, and somatosensory evoked potentials were consistent with a brainstem origin for the myoclonic jerks. Following ipsilateral digital and supraorbital electrical stimulation, the earliest muscle activation occurred in the trapezius. An all night sleep study showed frequent myoclonic jerks during sleep and markedly abnormal sleep architecture. We believe that this patient's myoclonus was related to pathologic changes in brainstem reticular nuclei that occurred as part of his disease process. To our knowledge, brainstem myoclonus has not been described as a feature of parkinsonian syndromes.