The risk of seizure disorders in relatives of patients with childhood-onset epilepsy was evaluated by a cohort study of the other decendants of parents of probands with epilepsy. The probands comprised 196 cases of idiopathic epilepsy and 60 individuals with isolated idiopathic seizures diagnosed among residents of Rochester, Minnesota, from 1935 to 1974. The risk of epilepsy through age 20 among the siblings and children of probands was 4.1%, three times the rate in the general Rochester population. The risk of any type of seizure (including isolated seizures, febrile convulsions, or seizures due to an acute cerebral insult) through age 20 was 11% in siblings and 13.1% in children of probands. The risks of seizure disorders among nieces and nephews of probands were not greater than the general population rates.