We report a study of 73 consecutive children with acute cerebellar ataxia, representing all of the children evaluated at St. Louis Children's Hospital during a 23-year-period to whom this diagnosis could appropriately be assigned. Twenty-six percent had chickenpox, 52% had other illnesses that were presumed to be viral, and in 3% the ataxia was related to immunization. Nineteen percent had no definite prodrome. Sixty children were followed for 4 months or longer after onset of their ataxia (mean, 7.4 +/- 6.0 years). Ninety-one percent (55/60) of these, including all children with chickenpox, recovered completely from ataxia. Eighty-nine percent (39/44) of the children with non-varicella-related ataxia recovered completely from the ataxia, a much better rate of recovery than what was found in prior large studies. One fifth of the children followed for more than 4 months experienced transient behavioral or intellectual difficulties, but only 5 of the 60 children demonstrated sustained learning problems. This study represents the largest reported series of acute cerebellar ataxia and the most complete characterization of the clinical features and outcome of this illness.