Clinical electrophysiological and neuroradiological examination of a case of unilateral intention and action myoclonus is reported. Brachial arteriography revealed an angioma located mainly in the lateral portion of the right posterior thalamus. Clinical and electrophysiological findings suggesting involvement of nonthalamic structures were absent. Our case, together with some cases previously described, raises the possibility that volitional myoclonus may be related to a relatively circumscribed damage of some diencephalic and/or mesencephalic structures. The rare occurrence of a hyperkinetic syndrome as principal symptom of a cerebral vascular malformation should be borne in mind in view of the potential risk of a stereotactic surgical procedure designated to alleviate the involuntary movements.