A 38-year-old woman had an episode of headache, fever and convulsion in October, 1993. She became alert in two weeks, though weakness and atrophy remained in the upper limb muscles. She was diagnosed as Russian spring summer encephalitis (RSSE) based on several serological studies. Three kinds of involuntary movements were noted after recovery. These were spontaneous muscle jerks in the left arm, action tremor in the right arm and pathological associated-movement in the right leg. The movements were studied physiologically by electroencephalogram (EEG)-electromyogram (EMG) polygraphic recordings, jerk-locked averaging (JLA), magnetencephalography (MEG), and sensory evoked potentials (SEPs). The jerky movements in her left arm were observed at rest and aggravated by emotional stress. EEG-EMG polygraph showed that the jerks were sometimes associated with small spikes thus were considered to be due to epilepsia partialis continua (EPC). JLA analysis revealed a pre-myoclonus spike on the right hemisphere which preceded the onset of the jerk in the left first dorsal interosseous muscle by 25.2ms, which was equal to the magnetic cortical latency of that muscle. Jerk-locked magnetic field, obtained by averaging neuromagnetic activities with respect to the onset of myoclonus, showed that the spike preceding the jerk, originated from the right motor cortex. The spontaneous spikes were localized mainly on the right motor cortex. Sensory evoked potentials (SEPs) were normal in both size and latency. No hyperexcitability of the sensory cortex was demonstrated even by using paired stimulation SEPs. Based on these results, we conclude that the jerky movements in this patient were produced by abnormal spontaneous discharges in the motor cortex. The action tremor had a frequency of 4-5 Hz and its clinical features were compatible with cerebellar tremor. This is thought to result from a lesion in the left thalamus, affecting the cerebellothalamic pathways. Her right leg unintentionally moved whenever she began to move the hands. This was considered to be a pathological associated-movement due to dysfunction of the pyramidal tracts.