Huntington's disease(HD) is clinically diagnosed by the triad of autosomal dominant inheritance, involuntary movements mainly chorea, and dementia. The phenotype of HD is variable and other diseases can have the same phenocopy. A definite diagnosis of Huntington's disease cannot be made by clinical informations alone Pathologic or genetic studies was necessary to exclude other neurodegenerative diseases which may present with familial dementia or chorea. Therefore, genetic studies of HD become essential for confirmatory diagnosis. Recent discovery of an expanded CAG trinucleotide repeat at the telomeric position of chromosome 4p made the diagnosis possible even in sporadic and presymptomatic cases. We previously demonstrated expantion of CAG repeats in clinically diagnosed HD, and were able to find presymptomatic. We herein present the clinical and genetic information in all the cases of genetically confirmed HD. 1) There was a clear gap between the number of CAG repeats in HD and normal and disease control. 2) Two out of three patients who had chorea without family history were confirmed as HD by genetic study. 3) One who had psychosis and a family history of HD was shown not to be HD. 4) We found 12 asymptomatic cases with HD mutation during family screening. 5) Caudate atrophy in MRI was not seen in the early stage of HD. Our data confirms that gene analysis is a powerful tool to make a diagnosis of HD even in sporadic and presymptomatic cases. Proper genetic counselling after judicious preparation of the family and society is sorely needed.