Methylphenidate is commonly believed to lower seizure threshold. The safe use of methylphenidate has not been clarified in patients with attention-deficit hyperactivity disorder (ADHD) and concomitant active seizure or electroencephalographic (EEG) abnormalities. Patients with ADHD and active seizures ( n = 57) and patients with ADHD and EEG abnormalities ( n = 62), 6 to 16 years of age, were included in the study. The safety and efficacy of treatment with antiepilepsy drugs combined with methylphenidate were determined by assessing seizure frequency, changes in ADHD symptoms, the Conners' Rating Scales, EEG differences, and side effects. The Conners' Rating Scales, performed by parents and teachers, and mean total ADHD symptom scores at the beginning of the study and at the end were significantly different ( P = .05 for the Conners' Rating Scales and P = .001 for ADHD symptom scores). Methylphenidate had a beneficial effect on EEG. Seizure frequency did not change from baseline. The side effects of methylphenidate were mild and transient. Methylphenidate is safe and effective in children with ADHD and concomitant active seizures or EEG abnormalities. ( J Child Neurol 2003;18:109—112).