The medication used in the study was Osmotic Release Oral System® (OROS) methylphenidate, to ensure homogeneity of the results. The specialist (neurologist or psychiatrist) adjusted the dose and monitored the effects. To ensure an effective response, drug treatment was adjusted for each child. Doses ranged from 18mg to 36 mg taken once a day. None of the children was taking any concomitant medication. This research was approved by the local ethics committee. All participants enrolled in this study provided written informed consent