Pharmacokinetics of Two Extended-Release Formulations of Methylphenidate in Children With Attention Deficit Hyperactivity Disorder (ADHD)

Inclusion Criteria: * The patient and the parents/authorized legal representatives must understand the nature of the study and be able to comply with protocol requirements. * Male patients aged 8-14 with Tanner stages 0 to 2 and a BMI between the 10th and 90th age percentile * Patients having a diagnosis of ADHD of any type according to DSM-IV criteria, as established by history, psychiatric examination and a structured diagnostic interview (Kiddie-SADS-Present and Lifetime Version) * Patients, whose symptoms are adequately controlled by a stable and well-tolerated dose of immediate release methylphenidate equivalent of 15 mg to 30 mg for at least one month before screening. * Patients with parents or a legal guardian, who will give written informed consent for the child to participate in the study. Additionally, assent to participate must be obtained from all children entering the study. Assent will be documented by the child's signature on the consent form. * Health status: Patients must have no clinically significant diseases or clinically significant abnormal laboratory values as assessed during medical history and physical exam. * Patients meeting minimum intelligence requirements: In the opinion of the investigator the patient must generally be functioning at age-appropriate levels academically, which should take into account any prior cognitive or academic testing (basic knowledge of reading, writing and calculating). * Patients already receiving behavioral therapies for ADHD may continue to do so during the course of the trial. Exclusion Criteria: * Patients with co-morbid psychiatric conditions with symptoms requiring current pharmacological treatment (e.g. major depression, psychosis). * Patients with co-morbid psychiatric or somatic conditions that may contraindicate treatment or confound efficacy or safety assessments. * Patients who are taking any concomitant medications likely to interfere with the study drug or confound efficacy or safety assessments, e.g. Tricyclic antidepressants, SSRIs except Fluoxetine, bupropion, clonidine, buspirone 2 weeks before randomization; Atomoxetine 2 weeks before randomization; Fluoxetine or antipsychotics 1 month before randomization; Pemoline and amphetamines 1 week before randomization. * Patients with a known non-response to methylphenidate. * History of hypersensitivity to any of the study drugs or to drugs with similar chemical structures. * Patients who are judged by the investigator as likely to be non-compliant with study procedures, including those with a suspected history of substance abuse, or patients living with a person diagnosed with a substance abuse disorder. * Use of other investigational drugs at the time of enrollment, or within 30 days or 5 half-lives of enrollment, whichever is longer * Patients with warnings, mentioned in the German Basic Prescribing Information of Ritalin LA (SmPC in the current version) or Medikinet retard (SmPC in the current version): anorexia, severe depression, anxiety disorder, Gilles de la Tourette-Syndrome, other tic disorder, hypertension, occlusive arterial diseases, severe stenocardia, tachycardiac arrhythmia, stroke, hyperthyroidism, increased intra-ocular pressure, hypertrophy of the prostate, known hypersensitivity to sympathomimetics, MAO-inhibitors. * Patients with a history of seizure disorder. * History of malignancy of any organ system, treated or untreated, within the past 5 years whether or not there is evidence of local recurrence or metastases, with the exception of localized basal cell carcinoma of the skin.