The electroencephalograms of 80 families of epileptic patients were compared with those of 30 normal control families. Abnormal family EEGs were not only encountered in patients with centrencephalic seizures but also in patients with psychomotor epilepsy. One half of all patients with focal sharp wave or spike discharges were found to have another family member with an abnormal EEG. Abnormal family EEGs were more frequently encountered in female than in male patients. Mothers of epileptic patients had a significantly higher percentage of electroencephalographic abnormalities than fathers, regardless of the type of seizure the patient was experiencing. Patients whose seizures started between the ages of 6 and 15 years usually had a genetic component to their illness, as demonstrated by electroencephalographic abnormalities in one parent and at least one sibling.