To clarify the mechanism of periodic dystonic myoclonus in subacute sclerosing panencephalitis (SSPE), a 22-year-old patient with a clinical diagnosis of SSPE was electrophysiologically investigated. Involuntary movements consisted of generalized dystonic posturing which occurred quasiperiodically once every 4 to 8 seconds. Effects of sensory stimuli and voluntary movements were studied by means of polygraphic recording of surface electromyogram (EMG), scalp electroencephalogram (EEG), and magnetoencephalogram (MEG). EEG showed quasi-periodic, generalized, transient complexes synchronous to each dystonic myoclonus, which were preceded by a slow negative EEG shift at the parietal region by approximately 5 seconds. Neither external stimuli nor self-paced movements alone influenced the periodicity of dystonic myoclonus or EEG complexes. In the reaction time task, however, the external stimuli given as an imperative cue to execute a motor task elicited dystonic myoclonus and generalized EEG complexes only if they were presented in the latter segment of the interval between the two successive EEG complexes while the slow negative EEG shift appeared. These findings suggest that EEG complexes and periodic movements spontaneously occur when cortical excitability reaches a certain critical level, but both phenomena are elicited even before if the sensory stimuli as an imperative signal requiring motor execution are presented. This finding most likely implies involvement of the sensorimotor integration mechanism in these periodic phenomena.