Reports concerning approximately 1900 extramural patients have been analyzed with respect to the present mental state of these patients and the determining influence of heredity on their mentality. Among these patients 64 per cent were mentally normal and only 14 per cent definitely deteriorated, in spite of the fact that the average patient had experienced 2000 seizures during a period of 8 years. The relatives of epileptic patients are more affected by mental disorders of various sorts than the normal population. Patients with essential epilepsy have more relatives with both epilepsy and neuropsychiatric disorders than patients with symptomatic epilepsy. The difference is more pronounced for fathers than for other members of the patient's family.Patients who were mentally abnormal at birth had approximately twice as many relatives with epilepsy and with psychosis as patients who were normal at birth. Patients whose mentality was normal at birth but is now abnormal have only a few more epileptic relatives than patients whose mentality has been persistently normal. Whatever the mentality of the patient at birth, the earlier in life seizures appeared, the greater was the number of relatives with epilepsy.These data indicate that heredity plays a rôle in the impaired mentality of certain patients subject to seizures. This rôle relatively small if patients were mentally normal before the seizures began. Other causes of mental impairment will be discussed in subsequent communications.Abstract Teaser