Double-blind, Placebo-controlled, Randomised Withdrawal, Extension, Safety and Efficacy Study of LDX in Children and Adolescents Aged 6-17

Inclusion Criteria: 1. Subject's parent or legally authorised representative(LAR) must provide signature of informed consent, and there must be documentation of assent (if applicable) by the subject indicating that the subject is aware of the investigational nature of the study and the required procedures and restrictions in accordance with the International Conference on Harmonisation (ICH) Good Clinical Practice (GCP) Guideline E6 and applicable regulations before completing any study-related procedures. 2. Subject and parent/LAR are willing and able to comply with all the testing and requirements defined in this protocol, including oversight of morning dosing. Specifically, the parent/LAR must be available upon awakening, at approximately 7:00AM, to dispense the dose of test product for the duration of the study. 3. Subject is a male or female aged 6-17 years inclusive at the time of consent for the antecedent study (SPD489-325). 4. Subject satisfied all entry criteria for the antecedent study (SPD489-325), and completed a minimum of 4 weeks of double-blind treatment, reached Visit 4 and completed the 1-week post-treatment washout in the antecedent study (SPD489-325), without experiencing any clinically significant AEs that would preclude exposure to LDX. 5. Subject must have a satisfactory medical assessment with no clinically significant or relevant abnormalities as determined by medical history, physical examination findings and clinical laboratory test results. 6. Subject has blood pressure measurements within the 95th percentile for age, gender, and height. Exclusion Criteria: 1. Subject was terminated from SPD489-325 for non-compliance and/or experienced an SAE or AE resulting in termination from the antecedent study. 2. Subject has a current, controlled (requiring a restricted medication) or uncontrolled, comorbid psychiatric diagnosis with significant symptoms such as any severe comorbid Axis II disorder or severe Axis I disorder (such as Post Traumatic Stress Disorder, psychosis, bipolar illness, pervasive developmental disorder, severe obsessive compulsive disorder, severe depressive or severe anxiety disorder) or other symptomatic manifestations, such as agitated states, marked anxiety, or tension that, in the opinion of the examining clinician, will contraindicate treatment with LDX or confound efficacy or safety assessments. Comorbid psychiatric diagnoses will be established at the Screening Visit (Visit -1)of the antecedent study (SPD489-325)with the Screening interview of the Kiddie-SADS-Present and Lifetime-Diagnostic Interview (K-SADS-PL)and additional modules if warranted by the results of the initial interview. Participation in behavioural therapy is permitted provided the subject was receiving the therapy for at least 1 month at the time of the Baseline Visit (Visit 0) of the antecedent study (SPD489-325). 3. Subject has a conduct disorder. Oppositional defiant disorder is not exclusionary. 4. Subject has any concurrent chronic or acute illness or unstable medical condition that could confound the results of safety assessments, increase risk to the subject or lead to difficulty complying with the protocol. 5. Subject is currently considered a suicide risk, has previously made a suicide attempt or has a prior history of, or is currently, demonstrating active suicidal ideation. 6. Subject is female and is pregnant or lactating. 7. Subject has glaucoma. 8. Subject has any clinically significant ECG at Visit 8 of the antecedent study (SPD489-325) or clinically significant laboratory abnormalities at Visit 7 of the antecedent study (SPD489-325). 9. Subject has a documented allergy, hypersensitivity, or intolerance to amphetamine. 10. Subject has a recent history (within the past 6 months) of suspected substance abuse or dependence disorder (excluding nicotine)in accordance with the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition - Text Revision (DSM-IV-TR)criteria. 11. Subject has a history of seizures (other than infantile febrile seizures), a tic disorder, a current diagnosis and or a known family history of Tourette's Disorder. 12. Subject has a known history of symptomatic cardiovascular disease, advanced arteriosclerosis, structural cardiac abnormality, cardiomyopathy, serious heart rhythm abnormalities, coronary artery disease, or other serious cardiac problems that may place them at increased vulnerability to the sympathomimetic effects of a stimulant drug. 13. Subject has a known family history of sudden cardiac death or ventricular arrhythmia. 14. Subject is taking any medication that is excluded. 15. Subject is taking other medications that have central nervous system (CNS) effects, affect performance, such as sedating antihistamines and decongestant sympathomimetics, or are monoamine oxidase inhibitors (during or within 14 days of investigational medicinal product administration). Stable use of bronchodilator inhalers is not exclusionary. 16. Subject has a documented allergy, hypersensitivity, or intolerance to any excipients in the investigational medicinal product(s).