The genetic locus of spinocerebellar ataxia type 3 (SCA3) is linked to chromosome 14q 24.3-qter like Machado-Joseph disease (MJD). The number of CAG repeats on mutant chromosome correlates positively with severity of disease, and negatively with the age of onset. However, the interindividual variation cannot be explained solely by the size of CAG repeats. We experienced a patient of juvenile onset SCA3, who had a relatively small length of CAG repeats. Several factors may have contributed in determining the age of on set in our case. The normal allele or modifying gene at other loci may have relationship with the age of onset and phenotype. Also, it should be considered that size of the expanded repeat in lymphocyte could be different from the size in cells of involved structures.