We have studied greatly enhanced cortical responses to somatosensory stimuli [giant somatosensory evoked potential (SEP) or magnetic field (SEF)] and cortical activities preceding the myoclonus (premyoclonus spike) in patients with cortical myoclonus, using magnetoencephalographic (MEG) techniques. A P1m component of giant SEF was estimated as a dipole positioned on the postcentral gyrus in all giant SEFs. This indicates that abnormally enhanced responses to sensory stimuli originate from the sensory cortex. In most of the patients with cortical reflex myoclonus, a dipole for the premyoclonus spike was also localized on the postcentral gyrus, which suggests that abnormal activation of the sensory cortex produces the spontaneous myoclonus. In one patient with galactosialidosis, two dipoles, one on the postcentral and the other on the precentral gyrus, could reasonably explain the premyoclonus spike. In the other patient with cortical reflex myoclonus, the premyoclonus spike associated with jerks of the lower limb muscle was estimated to be one dipole on the superior frontal gyrus just anterior to the paracentral lobule. This suggests that both spontaneous myoclonus and enhanced long loop reflex are generated by abnormal activation of the motor cortex in this patient. In a patient with epilepsia partialis continua who had no long loop reflexes, the premyoclonus spike was demonstrated to be positioned on the precentral gyrus, which indicates that abnormal activation of the motor cortex causes the spontaneous myoclonus in this patient. Our MEG studies demonstrated that abnormalities of the sensory or motor cortices contribute differently to the generation of myoclonus in cortical myoclonus, despite the fact that the sensory cortex is the main contributor in most of them.