The proportion of children with attention-deficit-hyperactivity disorder and epileptiform abnormalities is compared with an historic control group of normal school-aged children. The medical records of 655 children 5-16 years of age referred to a single pediatric neurologist (M.S.) from January 1991 to December 1999 with school problems, behavior problems, or hyperactivity were retrospectively reviewed. Clinical criteria for attention-deficit-hyperactivity disorder were satisfied in 476 of these children. An electroencephalogram was obtained from 347 patients and coded as epileptiform in 6.1 +/- 1.3%, which is significantly higher (chi-square test, P < 0.025) than the prevalence rate of 3.5 +/- 0.6% observed in a study of 3,726 normal school-aged children. The epileptiform abnormality was present only with activation procedures in six of our patients (hyperventilation [n = 2] and photic stimulation [n = 4]). Only three of the 21 children with epileptiform abnormalities developed a seizure disorder in our cohort. We conclude that the prevalence rate of epileptiform abnormalities is greater in children with attention-deficit-hyperactivity disorder compared with that observed in normal school-aged children when hyperventilation and photic stimulation are used. However, the clinical utility of routine electroencephalography in the diagnosis of a comorbid seizure disorder in children with attention-deficit-hyperactivity disorder is limited.