Spinocerebellar ataxia, type 3 (SCA3) and Machado-Joseph disease (MJD) are two clinically distinct representatives of the heterogeneous group of autosomal dominant cerebellar ataxias. Assignment of the disease genes to the same region of the long arm of chromosome 14 in both SCA3 and MJD suggested that these two disorders are genetically identical. The recent identification of a trinucleotide (CAG) repeat expansion in a gene underlying MJD facilitates assessment of this hypothesis. We analysed the MJD gene in members of a family with characteristic features of SCA3 and no symptoms typical of MJD. We found the same trinucleotide repeat expansion within the gene that was previously described in patients with MJD. The findings demonstrate that SCA3 and MJD are genetically identical in spite of their pronounced clinical differences. Furthermore, we demonstrate a striking variation in the copy number of the CAG repeat among affected members of the same family.