We reported a girl with "prolonged" cerebellar ataxia for whom steroid was effective. At the age of 9 months, she developed gait disturbance, tremor and abnormal eye movements following exanthema subitum. Her symptoms were prolonged for more than 4 months and she was admitted to our hospital. The symptoms were successfully suppressed with repeated ACTH treatment but recurred in a few weeks after cessation of the therapy. Steroid was also effective but reduction of the dosage resulted in worsening of symptoms. Immunological mechanism was suspected for her disorder because of her response to steroid and ACTH.