-A history of seizures was obtained for 32% of the 20,000 near relatives of 4231 epileptic patients. The incidence was 3.6% if evidence of brain damage prior to the patient''s 1st seizure was lacking, and 1.8% if such evidence was present. These percentages are, respectively, 7 and 3.5 times the incidence of epilepsy among draftees for the United States Army. As additional evidence of a genetic factor, in. the group with undamaged brain (essential epilepsy) the incidence of epilepsy among relatives decreased progressively with a later onset of seizures (7.6% if onset was in infancy; 1.5% if it was after the age of 30). Corresponding incidences for the group with brain damage (symptomatic epilepsy) were 2.9 and 1.3%. The relative influences of heredity, brain damage and chronicity of seizures were studied in a group of 122 twin pairs affected with seizures. In twin pairs without prior brain damage, both co-twins were epileptic in 84% of the one-egg and in 10% of the two-egg twins. In pairs with brain damage, the corresponding incidences were 17 and 8%. In one-egg twins, concordance was usual not only with respect to seizures, but also with respect to type of seizure and to electroencephalographic pattern. Intelligence quotients were detd. for 87 twin pairs. The person''s mental endowment was of most importance in the maintenance of a good intelligence. Acquired brain damage was a severe depressant; the reputed bad effect of repeated seizures was not in evidence. A transmitted predisposition to seizures and brain damage are each (or both) important factors in the origin of a person''s epilepsy. A constitutional cerebral dysrhythmia may, among other things, be the visual representation of a predisposition to seizures. The genetic factor in epilepsy is probably no greater than it is in many other common diseases. The epileptic is unfairly pillorized in public opinion and in law. Assets that are transmissible, such as sound vital organs, good intelligence, personality and social responsibility, may outweigh the liability of a tendency to seizures. Hence, advice regarding marriage and children must be individualized.