A survey of 15 families in the USA with Baltic myoclonus epilepsy showed that the 27 individuals who were affected had the following clinical picture from about the age of 10: photosensitive, occasionally violent, myoclonus, usually worse upon waking; generalised tonic-clonic seizures, sometimes associated with absence attacks; and light-sensitive, generally synchronous, spike-and-wave discharges on EEG that preceded clinical manifestations. Necropsy revealed marked loss of Purkinje cells of the cerebellum, but no inclusion bodies. Since the disease was confined to sibs and consanguinity was present in two families, autosomal recessive inheritance is probable. The disease progressed more rapidly in these families than it did in the early cases, seen in the Baltic region. This difference could be due to a toxic effect of phenytoin because phenytoin given alone or with other antiepileptic drugs was associated with progressive motor and intellectual deterioration, marked ataxia, and even death. Treatment with valproic acid, and the concomitant reduction or elimination of phenytoin, has been associated with marked improvement in at least 8 patients. Baltic myoclonus epilepsy must be distinguished from Lafora body disease, which is invariably fatal and discernible on clinical grounds.