Hereditary paroxysmal ataxia is a rare dominantly inherited disorder characterized by recurrent attacks of cerebellar ataxia, dysarthria, and nystagmus. Each attack lasts from several minutes to few hours or days. Usually there are no motor difficulties between attacks. We report a patient who had had recurrent ataxic episodes since early childhood. Four members of the family over two generations had similar attacks. There were no abnormalities in the laboratory studies including plasma amino acid, lactate, pyruvate, and EEG. Treatment with acetazolamide resulted in complete abolition of the attacks. Because of its dramatic response to acetazolamide, the recognition of this rare disorder is important.