Cortical myoclonus is associated with cortical activity preceding the myoclonus. In order to estimated the generator of this activity, we studied patients with cortical myoclonus using 37-channel first-order DC-SQUID gradiometer systems (Magnes, BTi Co Ltd).The subjects were seven patients with cortical reflex myoclonus with giant somatosensory evoked potentials (SEPs) and one with epilepsia partialis continua (EPC) due to Russian spring summer encephalitis who had no giant SEP. Neuromagnetic activity preceding the myoclonus were investigated by a backward averaging program triggered by the onset of myclonus (jerk-locked averaged magnetic field [JLF]).In six out of seven patients with cortical reflex myoclonus, JLFs were estimated as a single dipole localized on the sensory cortex. Their peak preceded the onset of myoclonus by the cortical latency of the jerking muscle. In the other patient with cortical reflex myoclonus, JLF was estimated as two dipoles. One of them was localized on the sensory cortex, and the other on the motor. In the patient with EPC, JLF was localized only on the motor cortex. Its first peak was localized on the pre-motor cortex and the second on the primary motor cortex. We conclude that there are three types of cortical myoclonus: 1. Hyperactivity of the sensory cortex mainly generates the myoclonus. 2. Hyperactivity of both the sensory and motor crotex relates with production of the myoclonus. 3. Only hyperactivity of the motor cortex causes the myoclonus.