Spinocerebellar ataxia type 8 (SCA8) is a neurodegenerative disorder characterized by slowly progressive cerebellar ataxia. It is caused by bidirectional expression of (CUG)n expansion in the ATXN8OS/ATXN8 gene and (CAG)n expansion transcripts in ATXN8. The diagnosis of SCA8 must be confirmed by the presence of a (CTG)n trinucleotide repeat expansion in the ATXN8OS gene. On the other hand, there are many human genetic diseases that are caused by expansion of short tandem repeats. Since Werner et al proposed a repeat-primed fluorescent PCR to detect large CTG-repeats in myotonic dystrophy, Friedreich ataxia, SCA2, SCA7, SCA10 and SCA12 have been reported. In this study, we applied a fluorescent PCR method for detection of expanded repeats in the ATXN8OS/ATXN8 gene. Although this test cannot give a precise estimate of the size of the expansion, it proved useful for confirming the presence of expansions in SCA8.