Objective: The most commonly reported side effects of methylphenidate, which is generally used for attention deficit hyperactivity disorder, are loss ofappetite, decrease of body weight and initial growth retardation. Ghrelin, which is dominantly released by the stomach, promotes feeding, decreases energy expenditure and locomotor activity, enhances weight gain and fat mass deposition and also effects gastrointestinal,motility. Ghrelin may be related to the metabolic and anorexigenic effects of methylphenidate in children. The aim of this study was to investigate methylphenidate's effect on fasting serum active ghrelin levels in prepubertal children with attention deficit hyperactivity disorder. We expected to find a difference between pre- and post-treatment ghrelin levels with 18 mg/day methylphenidate administered via an osmotic-controlled release oral delivery system in prepubertal boys.