Issues relating to the definition and classification of ADHD are outlined from the Departments of Pediatrics, Neurology, and Child Study, Yale University School of Medicine, New Haven, CT. In DSM-III-R (1987) attention deficit disorder with hyperactivity was referred to as ADHD, and ADD without hyperactivity was called undifferentiated ADD. In DSM-IV (1994) the categories of ADHD are 1) inattention only, 2) hyperactive only, or 3) combined inattention-hyperactive type. “Attention,” the psychological construct as measured in the laboratory, should not be confused with “behavior attention deficit,” the disorder evaluated by rating scales. Despite the DSM definitions of types, children with ADHD represent a heterogeneous population that varies with 1) the degree of cognitive and behavioral overlap, 2) the relative predominance of inattention or hyperactive-impulsive behavior, and 3) the specialty interest of the professional who diagnoses and treats the patient - pediatrician, child neurologist, psychiatrist, psychologist, educator, or speech-language pathologist. Samplings from mental health settings have different characteristics compared to those from pediatric neurology clinics. Patients with comorbid behavioral disorders such as oppositional and conduct disorders would be referred to psychiatrists and clinical psychologists, while those with ADHD complicated by learning disabilities are more likely to be seen by the pediatric neurologist and educational psychologist. A systematic classification of subtypes of ADHD should lead to more precise definitions of etiology, treatment, and prognosis. 
COMMENT. The neurologic and psychiatry examinations, psychologic evaluations, and EEG and evoked potentials are important in the differentiation and treatment of subgroups of ADHD children and adolescents. (see Millichap JG, Ed, Progress in Pediatric Neurology I, Chicago, PNB Publ, 1991; and Ped Neur Briefs March 1995;9:20). Abnormalities of CNS maturation and function revealed by longitudinal auditory evoked responses and EEGs were found to characterize non-delinquent ADHD subjects, while delinquent hyperactive subjects showed normal CNS maturation. ADHD boys with neurologic abnormalities had a better outcome than those with normal CNS function who later exhibited delinquent behavior secondary to environmental social factors, (see PPN I, ppl59-160).