Trinucleotide repeat expansion in the Machado-Joseph disease (MJD) gene has been found in 26 patients from 20 unrelated Japanese families. Expanded alleles had 68 to 84 repeats, whereas normal alleles had 14 to 37 repeats. The age of onset was inversely correlated with the repeat length. To evaluate in detail the relationship between the repeat length and clinical features, we subdivided the 26 patients into three groups on the basis of the repeat length (group 1, 78 repeats or more; group 2, 74 to 77 repeats; group 3, 73 repeats or less). Group 1 and group 2 had common features of spasticity, hyperreflexia, Babinski sign, bulging eyes, facial myokymia and extrapyramidal signs as well as cerebellar ataxia and ophthalmoplegia. It should be noted that group 1 showed more prominent pyramidal and extrapyramidal signs than group 2. In contrast, group 3 showed hypotonia, hyporeflexia and sensory disturbance in addition to cerebellar ataxia and ophthalmoplegia. These findings suggest that the repeat length plays an important role in phenotypic variation. DNA analysis for the MJD mutation was clearly useful for making an accurate diagnosis in patients without bulging eyes, facial myokymia, dystonia or marked spasticity.