Epileptic negative myoclonus (ENM) is an epileptic motor dysfunction characterised by brief lapses of postural tone provoked by paroxysmal cortical discharges. We report the electrophysiological and video-polygraphic findings in a patient presenting with the unusual association of ENM and brief asymmetric tonic-postural seizures of cryptogenetic origin. Focal EMG silent periods in the left deltoid muscle (mean duration 81.2 +/- 16.4 ms), time-locked with vertex spikes were present. The time lag between spikes and ENM was 33.1 +/- 4.6 ms. We suggest the involvement of mesial frontal areas in the genesis of both negative and positive motor phenomena.