EEG RECORDINGS WERE PERFORMED ON 22 HOSPITALS PATIENTS WITH SEX CHROMOSOME ABNORMALITIES AND A CONTROL GROUP, ALSO PATIENTS AT THE HOSPITAL, BUT DISPLAYING NORMAL KARYOTYPES AND MATCHED FOR AGE, INTELLIGENCE QUOTIENT, AND DURATION OF STAY. THE SUBJECTS WITH SEX CHROMOSOME ABNORMALITIES HAD A HIGHER PREVALENCE OF EEG DISTURBANCE. THE CHANGES OBSERVED ALMOST INVARIABLY INVOLVED THE BACKGROUND ACTIVITY AND CONSISTED OF AN INCREASED FREQUENCY OF UNDULY SLOW DOMINANT RHYTHMS, SLOW ALPHA RHYTHM, AND A GENERALIZED EXCESS OF THETA ACTIVITY. THE PREVALENCE OF EEG CHANGE WAS AS COMMON IN THE XYY AS IN THE XXY SYNDROME. TABULAR DATA AND REFERENCES ARE INCLUDED. (KBL)