This study evaluated the efficacy and tolerability in the initiation treatment of Concerta (OROS methylphenidate HCl) in Korean children with Attention-Deficit/Hyperactivity Disorder (AD/HD). One hundred and nineteen children with AD/HD were entered into a multi-center, open-label, four-week trial. The dosage of Concerta was adjusted by the investigators based on symptoms and safety assessments performed on a weekly basis. The safety of the drug and its efficacy for attention, behavior, and cognitive function were assessed. The primary outcome measures for efficacy were the Parent and Teacher IOWA Conners Rating Scales, Peer Interaction Items, and the Clinical Global Impression Scale. Cognitive tests (Continuous Performance Test, Matched Familiar Figure Test, Verbal Fluency Test, and Trail Making Test) were included as the secondary outcome measures. In most participants, OROS methylphenidate was well tolerated. There were significant improvements in attention, behavior, and function as measured by parents, teachers, and investigators. The benefit of the initiation of OROS methylphenidate in children with AD/HD was shown on the cognitive tests as well. These data provide support for the benefit of the once-daily methylphenidate preparation, Concerta in the treatment of Korean children with AD/HD. Children were initiated safely in this short-term trial, and its effectiveness was evident in the behavioral, as well as neuropsychological measurements.