Cerebellar ataxia was diagnosed in a 62-year-old woman, its signs regressing almost completely within six weeks during treatment with thyroxine and triiodothyronine. The cause of cerebellar ataxia in association with hypothyroidism remains unknown. No typical morphological changes in the cerebellum have been described. It is assumed that a thyrogenic, specific metabolic factor is responsible which aggravates already existing non-specific cerebellar changes. The prognosis is very good. On the other hand, cerebellar ataxia resulting from congenital hypothyroidism has typical histological cerebellar changes and prognosis is very poor unless thyroid treatment is started soon after birth.