Rhythmic myoclonus in an arm began abruptly following an injury and persisted continuously for six years. Topographical EMG showed abnormal activity confined to muscles innervated by the axillary and radial nerves from the posterior cord of the brachial plexus. Abduction of the arm above horizontal level stopped myoclonus and EMG discharges. EEG was normal. It is suggested that the myoclonus was caused by mechanical irritation of the posterior cord of the brachial plexus.