A 52-year-old man with clinically diagnosed corticobasal degeneration (CBD) who developed the asymmetric akinetic-rigid syndrome was studied by means of transcranial magnetic stimulation (TMS) after an interval of two years. TMS of the motor cortex during tonic muscle contraction causes a silent period immediately after a motor evoked potential in an electromyogram. This silent period is considered to reflect activity of an intracortical inhibitory mechanism. In this patient, the motor symptoms started from the left half of the body and spread to the right. The major neurological findings were dystonia, rigidity, clumsiness, and hyperreflexia. The involuntary contraction of the muscles, which was unrelated to voluntary movement, seemed to be responsible for his signs and symptoms. There was laterality of the threshold intensity and the duration of the silent period after a motor evoked potential; the threshold was higher and the duration was shorter on the affected side. Also, we found that the threshold increased and the duration decreased as his motor dysfunction progressed, which has not been reported before. A comparison of the clinical symptoms and the results of TMS in this case together with previous TMS data on CBD suggests that a disorder of the intracortical inhibitory mechanism is closely related to the movement impairment in CBD.