Recurrent encephalopathy affecting cerebellar and extrapyramidal structures was observed in five members of two families. The syndrome is characterized by sudden onset of truncal ataxia, occasionally accompanied by lethargy and impairment of speech. Choreic and athetoid movements were present, and there was loss of deep tendon reflexes with presence of pathological reflexes. Onset of the disease was early in childhood. Attacks lasted for days to weeks; residual symptoms comprising speech impairment and incoordination were seen in some patients. Both sexes were affected. The pedigrees suggest autosomal dominant inheritance. Pathogenesis remains unexplained by the laboratory studies done; metabolic or immunological processes predisposed by genetic factors are suggested. Similar reports from the literature are discussed; no identical family could be found.