The main objective of this study is to assess the prevalence of sleep disorders in treatment-naive children and adolescents with ADHD compared with healthy controls matched for age and gender; and, at the same time, to determine whether stimulant medication (methylphenidate) affects sleep in the same group of children and adolescents with ADHD (naive) after a 3-month treatment. A total of 120 children and adolescents (60 newly diagnosed with ADHD according to the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM IV), and 60 gender- and age-matched controls) were evaluated through objective (actigraphy) and subjective (sleep diary) measures. Later, of those 60 newly diagnosed with ADHD, 30 started pharmacological treatment with methylphenidate with a mean daily dose of 0.58 mg/kg. No significant differences in both measures within the ADHD group after following the 3-month pharmacological treatment with methylphenidate were found. There were neither any significant differences in sleep parameters through objective measures between the medication-naive ADHD group and the control group, while significant differences were found through sleep diary (registered by parents) in latency and efficiency (p =