Cerebellar degeneration has been demonstrated in several patients receiving phenytoin therapy. In most cases it has been unclear, however, whether the degeneration was caused by the drug per se or by other mechanisms known to lead to cerebellar damage. We describe a patient who developed a marked cerebellar atrophy, demonstrated on computed tomographic scan, following an episode of acute, severe phenytoin intoxication. The patient received phenytoin prophylactically for 2 1/2 months after an uncomplicated subarachnoid hemorrhage and was in good health when the treatment was started. He has never had seizures, and no other possible cause of chronic cerebellar changes is known. The initially severe clinical signs of cerebellar dysfunction have subsided slowly. We conclude that phenytoin can directly cause cerebellar degeneration.