The cerebellar variant of multiple system atrophy (MSA‐C) has overlapping clinical features with the hereditary spinocerebellar ataxias (SCAs), but can usually be distinguished on a clinical basis. We describe a patient who developed a sporadic, late‐onset, rapidly progressive neurodegenerative disorder consistent with MSA‐C. Genetic testing, however, showed an abnormal expansion of one allele of the spinocerebellar ataxia 3 (SCA3) gene. The clinical impression of MSA‐C was confirmed by identification of numerous α‐synuclein–containing glial cytoplasmic inclusions on autopsy. These findings suggest that abnormal expansion of the SCA3 gene may be a risk factor for the development of MSA‐C. © 2006 Movement Disorder Society