We examined the cerebral cortices of six brains from patients with sporadic olivopontocerebellar atrophy (s-OPCA), five control brains including four from patients who had died without neurological disease, and one from a patient with Holmes-type cerebellar cortical atrophy. Distinct laminar astrocytosis of the motor cortices in the fifth layer were demonstrated in 4 of 6 s-OPCA cases and in none of the control cases by immunohistochemistry for glial fibrillary acidic protein. The astrocytosis localized in the primary motor cortex and its distribution pattern were clearly different from those of so-called glial cytoplasmic inclusion. This cortical astrocytosis appears to be characteristic of s-OPCA and may reflect the pathology of the primary motor cortex.