Hereditary essential myoclonus is a disease in which segmental myoclonus is the sole clinical abnormality and whose cause is unknown. It is characterized by an early onset, a benign course, an autosomal dominant pattern of inheritance, the absence of any other neurologic dysfunction, and normal results of auxiliary tests. Cerebral blood flow studies of a father and son with this disease showed a cortical blood flow reduction contralateral to the myoclonus symptoms. We postulate the cause to be a focal unilateral subcortical cerebral lesion, either in the basal ganglia or in the brain stem, with subsequent cortical deafferentation.