• ADHD-Rating Scale (ADHD RS)- IV- Parent Version [ Time Frame: Measured at each visit ] [ Designated as safety issue: No ]
  
• Clinical Global Impressions (CGI)- Severity, Overall, Improvement [ Time Frame: Measured weekly throughout the study ] [ Designated as safety issue: No ]
  
• Conners Parent Rating Scale- Short Form [ Time Frame: Measured at screening, baselines one and two, and ends of treatments one and two ] [ Designated as safety issue: No ]
  
• Atomoxetine-Stimulant Side Effects Ratings Scale [ Time Frame: Measured at each visit ] [ Designated as safety issue: Yes ]
  
• Weiss Functional Impairment Scale [ Time Frame: Measured at screening, baselines one and two, and ends of treatments one and two ] [ Designated as safety issue: No ]
  

• Conners-Wells Adolescent Self Report [ Time Frame: Measured at screening, baselines one and two, and ends of treatments one and two ] [ Designated as safety issue: No ]
  
• Conners Teacher Rating Scale- Short [ Time Frame: Measured at screening, baselines one and two, and ends of treatments one and two ] [ Designated as safety issue: No ]
  
• Kiddie-Schedule for Affective Disorders and Schizophrenia-Present and Lifetime Version (K-SADS-PL) [ Time Frame: Measured at screening ] [ Designated as safety issue: No ]
  
• Psychiatric History and Mental Status Examination [ Time Frame: Measured at screening ] [ Designated as safety issue: No ]
  
• Child Behavior Checklist (CBCL) [ Time Frame: Measured at screening ] [ Designated as safety issue: No ]
  
• Social Skills Rating Scale (SSRS)- Parent Version [ Time Frame: Measured at screening, baselines one and two, and ends of treatments one and two ] [ Designated as safety issue: No ]
  
• Social Skills Rating Scale (SSRS)- Teacher Version [ Time Frame: Measured at screening, baselines one and two, and ends of treatments one and two ] [ Designated as safety issue: No ]
  
• Child Conflict Index (CCI) [ Time Frame: Measured at screening, baselines one and two, and ends of treatments one and two ] [ Designated as safety issue: No ]
  
• Wechsler Intelligence Scale for Children - 4th Edition (WISC-IV) [ Time Frame: Measured at screening ] [ Designated as safety issue: No ]
  
• Wechsler Individual Achievement Test II Abbreviated (WIAT II A) [ Time Frame: Measured at screening ] [ Designated as safety issue: No ]
  
• Continuous Performance Test (CPT) [ Time Frame: Measured at screening, baselines one and two, and ends of treatments one and two ] [ Designated as safety issue: No ]
  
• Permanent Mathematics Product Test (PERMP) [ Time Frame: Measured at screening, baselines one and two, and ends of treatments one and two ] [ Designated as safety issue: No ]
  
• Children's Sleep Questionnaire [ Time Frame: Measured at screening, baselines one and two, and ends of treatments one and two ] [ Designated as safety issue: No ]
  
• Actigraphy [ Time Frame: Measured daily throughout the study ] [ Designated as safety issue: No ]
  
• Sleep logs [ Time Frame: Measured daily throughout the study ] [ Designated as safety issue: No ]
  
• Hyperactivity, Attention, and Learning Problems (HALP) Medical and Developmental History Questionnaire [ Time Frame: Measured at screening ] [ Designated as safety issue: No ]
  
• Vital signs [ Time Frame: Measured at each visit ] [ Designated as safety issue: No ]
  
• Open Ended Adverse Event Questioning [ Time Frame: Measured at each visit ] [ Designated as safety issue: Yes ]
  
• Modified Yale Global Tic Severity Scale [ Time Frame: Measured at screening, baselines one and two, and ends of treatments one and two ] [ Designated as safety issue: No ]
  
• Assessment of Affective Range (AAR) [ Time Frame: Measured at screening, baselines one and two, and ends of treatments one and two ] [ Designated as safety issue: No ]
  
• Treatment Preference Survey [ Time Frame: Measured at ends of treatments one and two ] [ Designated as safety issue: No ]
  
• HALP Rebound Effects Questionnaire [ Time Frame: Measured at ends of treatments one and two and baseline two ] [ Designated as safety issue: No ]
  

ADHD is one of the most frequently occurring disorders of children and adolescents and is a significant public health problem. The most common treatment for the condition is stimulant medication. However, there are an increasing number of children who are experiencing negative side effects from stimulants, such as dizziness, loss of appetite, and headaches; these side effects have made the need for alternative treatments all the more important. This study will compare the stimulant methylphenidate to the nonstimulant atomoxetine to determine which is more effective in treating ADHD symptoms in children and adolescents. The two medications differ in the neurotransmitters they influence. Stimulants such as methylphenidate act upon the neurotransmitter dopamine, while atomoxetine works on norepinephrine. It has been proposed that the difference in neurotransmitter stimulation may result in differences in an ADHD patient's response to treatment.

Participants will be randomly assigned to receive either methylphenidate or atomoxetine for either 2 or 5 weeks, depending on how soon they respond to the treatment. After the 2-or 5-week period, participants will be crossed-over to receive whichever medication they did not receive in the first part of the study.

Participants will have up to 20 weekly study visits. Over the first two visits, participants will undergo psychological and intelligence tests, a medical history, an electrocardiogram, blood and urine collection, and a physical exam. The remaining visits will occur weekly. During these visits, participants will receive their assigned medication and, along with their parents, will complete questionnaires about their response to treatment and any side effects they may be experiencing. The teachers of all participants will be asked to complete a questionnaire about their student's behavior at 4 different times during the study. Participant, parent, and teacher questionnaires will be used to assess the ADHD symptoms of participants, as well as self-report clinical scales completed by the participants.