We prospectively studied the clinical and electrophysiological features of myoclonic tremulous movements in patients with multiple system atrophy (MSA). Among 42 consecutive patients, 12 MSA-p (parkinsonian type) and 3 MSA-c (cerebellar type) patients showed small-amplitude myoclonic movements. These movements occurred in the distal part of the arms and fingers, particularly in posture or during voluntary movements. We conducted detailed electrophysiological studies in 11 patients. Electromyographic recordings showed irregular bursts of brief duration often synchronously involving the antagonistic muscles. Somatosensory evoked responses were slightly enlarged in some patients. Long-latency reflexes were enhanced in 7 patients. A jerk-locked averaging technique revealed premyoclonic cortical potentials in 9 patients. These findings suggest that small-amplitude myoclonic movements in MSA are a form of cortical myoclonus.