Electroencephalographic (EEG) studies of 60 patients with bipolar manic-depressive disease disclosed an incidence of small sharp spikes plus a few other variations in 47% of the sample. In women these EEG features were significantly associated with a history of mental illness in the patient's mother or the maternal side of the family and an absence of mental disorder in the fathers. The reverse was true of women probands without these EEG characteristics. In the men small sharp spikes did not relate to parental psychopathology but half of the sisters of men with these EEG characteristics were found to be mentally ill. On the basis of these observations and previous work, we hypothesize that the small sharp spike EEG pattern might be an inherited characteristic related in some way to the familal transmission of manic-depressive disease.