Efficacy and Safety of Dex-Methylphenidate Extended Release 30 mg Versus 20 mg in Children (6-12 Years) With Attention-Deficit/Hyperactivity Disorder (ADHD) in a Laboratory Classroom Setting.

Inclusion Criteria: * Male and female subjects aged 6-12 years, inclusive. * Subjects meeting the DSM-IV criteria for primary diagnosis of ADHD-Combined type, or predominantly hyperactive-impulsive subtype, as established by the K-SADS-PL (Kiddie Schedule for Affective Disorders and Schizophrenia-Present and Lifetime Version). If a DSM-IV-defined ADHD diagnosis is difficult to establish due to possible co-morbidity, the subject will not be enrolled into the study. * Subjects should be on a stabilized total daily dose or nearest equivalent of 40-60 mg methylphenidate or 20-30 mg of d-methylphenidate for at least two weeks prior to Screening visit. Exclusion Criteria: * Subject or subject's guardian unable to understand or follow instructions necessary to participate in the study. * Diagnosed with or history of a tic disorder or Tourette's syndrome. * History of seizure disorder. * The presence of a known medical condition that would preclude the use of methylphenidate. * A history (within the past year) or presence of clinically significant cardiovascular, cerebrovascular, renal, hepatic, gastrointestinal, pulmonary, immunological, hematological, endocrine, or neurological disease. * ALT (Alanine Amino Transferase), AST (Aspartate Amino Transferase), GGT (Gamma glutamyl transferase) or serum creatinine greater then 2X the ULN (Upper Limit of Normal) at Screening. * A history of psychiatric illness or substance use disorder (e.g., schizophrenia, bipolar disorder, autism, abuse or dependence, depression, severe Conduct Disorder or severe Oppositional defiant disorder) * Subjects who have participated in an investigational trial within the past 4 weeks (28 days) * Subjects who are currently taking antidepressants or other psychotropic medication. * Subjects who have initiated psychotherapy during the three months prior to randomization. * Subjects with a positive urine drug screen. * Subjects who have a history of poor response or intolerance to methylphenidate or d-methylphenidate. Other protocol-defined inclusion/exclusion criteria may apply