Spinocerebellar ataxia type 8 (SCA8) is the first example of dominantly inherited ataxia reported to be caused by a dynamic mutation of the untranslated CTG trinucleotide repeat. We performed genetic and clinical analyses of a family with an isolated case with young onset cerebellar ataxia carrying an expanded 95 CTA/CTG repeats, and revealed that the asymptomatic father was also carrying a much greater expansion of 136 repeats. This paternal transmission developed a large contraction of -41 CTG repeats. The ataxia patient showed almost pure cerebellar symptoms, and a cerebral MRI of the patient demonstrated significant atrophy of the cerebellar vermis and hemispheres with preservation of brainstem and cerebrum. Although the father did not show any neurological abnormalities, his MRI demonstrated mild atrophy of the cerebellar hemispheres. The genetic phenomenon on this family has not been observed in other types of SCAs, and this reduced penetrance may cause reproduction of sporadic SCA8 frequently. Therefore, we must perform careful interviews regarding family history, and suggest the genetic and neuroradiological investigations on family members when we encounter a sporadic patient with the CTG expansion at the SCA8 locus.