This is a dose-optimized, randomized, double-blind, placebo-controlled crossover study in approximately 100 pediatric patients (aged 6 to 12 years) with ADHD. Eligible patients will enroll to take open-label AR11 BID and undergo dose optimization activities for 8 weeks. Patients who achieve a stable dose during the dose optimization period will continue participation and will be randomized to take double-blind medication (AR11 or placebo) orally twice daily for 1 week. At the end of each double-blind treatment period, patients will be evaluated for ADHD symptoms in a laboratory classroom setting utilizing SKAMP and PERMP assessments.

This is a dose-optimized, randomized, double-blind, placebo-controlled crossover study in approximately 100 pediatric patients (aged 6 to 12 years) with ADHD. Eligible patients will enroll in the study to take open-label AR11 twice daily and undergo dose optimization activities for 8 weeks. Patients who achieve a stable dose during the dose optimization period will continue participation and will be randomized to take double-blind medication (AR11 or placebo) orally twice daily for 1 week. At the end of the first double-blind treatment period, patients will be evaluated for ADHD symptoms in a laboratory classroom setting utilizing SKAMP and PERMP assessments. At the end of the second double blind treatment period, patients will be evaluated for ADHD symptoms in a second laboratory classroom setting utilizing SKAMP and PERMP assessments.

Inclusion Criteria:

1. Male or female between 6 and 12 years of age, inclusive, at the time of Screening.
2. Diagnosed as meeting Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR) criteria for ADHD.
3. A clinician-administered Clinical Global Impression of Severity (CGI-S) score of 3 or greater.
4. An ADHD Rating Scale (ADHD-RS) score at Screening and Baseline greater than or equal to the 90th percentile normative values for gender and age in at least one of the following categories: hyperactive-impulsive subscale, inattentive subscale, or total score.

Exclusion Criteria:

1. Secondary or co-morbid diagnoses other than ADHD, with the exception of simple phobias, oppositional defiant disorder, elimination disorders, motor skills disorders, communication disorders, learning disorders, adjustment disorders, and sleep disorders if, in the opinion of the investigator, the associated symptoms do not confound assessment of safety or efficacy.
2. Clinically significant cognitive impairment as assessed in the clinical judgment of the Investigator.
3. History of any of the following medical disorders: seizure disorder (excluding a history of febrile seizures), structural cardiac disorders, serious cardiac conditions, hypertension, untreated thyroid disease, glaucoma, Tourette's disorder, or chronic tics.
4. Clinically significant abnormal ECG finding or abnormal cardiac finding on physical exam (including presence of a pathologic murmur) at Screening.
5. Use of any psychotropic medication (sedative hypnotics prescribed as a sleep aid at a stable dose for at least 30 days prior to Baseline, at bedtime only, are allowed during the study).
6. A history of hypersensitivity or intolerance to any formulation of amphetamine or lisdexamfetamine.