In contrast to juvenile rigid form of Huntington's disease (HD) in which myoclonus is often seen, only 5 patients with myoclonus complicating adult HD have been reported. We herein described an adult HD patient who suffered from severe action myoclonus leading to physical disability. To our knowledge, this is the first case report in Japan. The patient, a 32-year-old female with a family history of chorea, developed choreiform movements and mental changes since the age of 24. Subsequently her motor disability has been aggravated by distinctively different involuntary movements characterized by sudden, violent, continuous muscular contractions of four extremities on any attempts at movement. Examination revealed moderate dementia and chorea complicated by frequent myoclonic jerks involving upper and lower extremities in posture or during movement. A head CT scan and MRI revealed caudate atrophy. The myoclonus, as recorded by surface electromyography over the right arm consisted of 40-60 msec-synchronous semirhythmic bursts. The cortical component of SEP was enlarged and C reflex was also observed. Clonazepam (4 mg a day) was instituted with a pronounced reduction in myoclonus and a return to her previous level of daily life activity. Although myoclonic jerks are often recognized in juvenile patients with rigid form of HD, they have been considered to exert a minor influence on physical disability. By contrast, our present observation and review of literature suggest that myoclonus may lead to severe motor impairment in adult HD.