Early onset torsion dystonia (DYT1) is a dominantly inherited dystonia caused by a deletion of three bases, GAG, coding glutamic acid, in chromosome 9q34. The protein coded by this gene was named as torsin A. DYT1 is common among the Ashkenazi Jewish population, but has been thought to be rare among Japanese. Among the idiopathic torsion dystonias being followed in this clinic, we found five families with DYT1 by gene analysis. This is the first report of genetically proven Japanese DYT1.The clinical features of five proband cases were divided into two types. One type is postural dystonia with marked trunkal torsion, and the other is action dystonia associated with violent dyskinetic movements. The affected family members in the upper generations presented with focal or segmental dystonia; it was postural dystonia of the legs in the former, and writer's cramp or tremor of the arms in the latter families. There was an asymptomatic carrier in the upper generation. Anticipation in the age of onset and severity of the disease was observed in all families. Medical treatment, including anticholinergics and levodopa, did not show apparent effects, while stereotactic thalamotomy to the nucleus ventralis lateralis (VL) or ventralis intermedius (Vim), with or without posterior ventral pallidotomy, were effective with action dystonia, but not postural dystonia. This study suggests the existence of at least two phenotypes in DYT1, in which different pathways of the basal ganglia are involved.