THE PROBLEM CHILDREN, WHOSE AGES RANGED FROM 5 TO 15 YEARS, HAD BEEN REFERRED TO AN OUTPATIENT CLINIC AT THE UNIVERSITY OF KENTUCKY. CONTROL SUBJECTS WERE CHOSEN FROM THE STUDENT BODIES OF NEARBY PUBLIC SCHOOLS. PSYCHOMETRIC TESTS, NEUROLOGICAL EXAMINATIONS, AND EEG'S (MEASURES OF THE BRAIN'S ELECTRICAL ACTIVITY) WERE COMPLETED FOR ALL THE CHILDREN. THERE WERE SIGNIFIANT DIFFERENCES BETWEEN THE TWO GROUPS IN ALL THREE MEASURES, AN INDICATION THAT THE PROBLEM CHILDREN WERE SUFFERING FROM BRAIN DYSFUNCTION. WITHIN THE PROBLEM GROUP, THERE WERE 11 DEFINITELY HYPERACTIVE AND 9 DEFINITELY NONHYPERACTIVE CHILDREN. TWO DISTINCT SYNDROMES WERE DISTINGUISHABLE; (1) A SYNDROME CHARACTERIZED BY HYPERACTIVITY, PERCEPTUAL-MOTOR DEFICITS, EXCESSIVE NEUROLOGICAL 'SOFT SIGNS' (UNCERTAIN RESPONSES IN NEUROLOGICAL EXAMINATIONS), AND EXCESSIVE, NONAGE-DEPENDENT SLOW ACTIVITY AND ABNORMAL TRANSIENT DISCHARGES (TYPES OF BRAIN ELECTRICAL ACTIVITY) ON THE EEG; AND (2) A SYNDROME CHARACTERIZED BY EXCESSIVE NEUROLOGICAL SOFT SIGNS, EXCESSIVE BUT AGE-DEPENDENT SLOW ACTIVITY AND ABNORMAL TRANSIENT DISCHARGES, BUT NO PERCEPTUAL-MOTOR DEFICITS OR HYPERACTIVITY. LONGITUDINAL STUDIES ARE NEEDED TO DETERMINE WHETHER THE CENTRAL NERVOUS SYSTEM ABNORMALITY IN HYPERACTIVE PROBLEM CHILDREN DIFFERS FROM THAT IN NONHYPERACTIVE PROBLEM CHILDREN IN KIND OR IN SEVERITY. SUPPORTING DATA AND A LIST OF REFERENCES ARE INCLUDED. (LKM)