Pharmacokinetic Study of Methylphenidate HCl Extended-Release Capsules in Children 4 to Under 6 Years of Age With ADHD

Inclusion Criteria: 1. Patient is a male or female between the ages of 4 and under 6 years old. 2. Patient has a history consistent with ADHD, meets the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) criteria for ADHD, inattentive, hyperactivity or combined. 3. Patient must meet criteria for ADHD diagnosis on Kiddie Schedule for Affective Disorders and Schizophrenia for School-Age Children-Present and Lifetime (KSADS-PL) and clinical interview by experienced clinician; symptoms must have been present for at least 6 months. 4. Subject has had prior behavioral treatment or subject's symptoms are severe enough to warrant treatment without prior behavioral treatment, and patient is on a stable dose of either immediate-release or extended-release methylphenidate. 5. Subject must have age- and sex-adjusted ratings of ≥ 90th percentile Total Score on the Attention Deficit Hyperactivity Disorder Rating Scale-IV (ADHD-RS-IV) Preschool Version, a Clinical Global Impressions -Severity Score of ≥4 and a Child Global Assessment Scale rating of \<65 after methylphenidate washout and prior to obtaining pharmacokinetic samples. Ratings may be completed via telephone on day-1. 6. Parents or guardians of patients must have the ability to read and understand the language in which the Informed Consent is written and are mentally and physically competent to provide written informed consents for their child. 7. Patient and/or parent are/is able to understand English in order to provide assent and is otherwise able to comply with the study protocol. Exclusion Criteria: 1. Patient has allergy to methylphenidate or amphetamines, or history of serious adverse reaction to methylphenidate. 2. Patient has a history of tension, agitation, glaucoma, thyrotoxicosis, tachyarrhythmias or severe angina pectoris or patient with serious or unstable medical illness such as asthma, diabetes or seizures. 3. A history of motor or vocal tics or Tourette's syndrome 4. Patient is receiving monoamine oxidase inhibitors, anticonvulsants (phenobarbital, phenytoin, primidone), coumarin anticoagulants, presser agents, guanethidine, tricyclic antidepressants (imipramine, desipramine, selective serotonin inhibitors (SSRIs), or herbal remedies (e.g., melatonin). 5. Patient has serious hypertension. 6. Patient has a history of disorders of the sensory organs, particularly deafness, severe or profound retardation. 7. Patient has any other unstable psychiatric condition requiring treatment. 8. Patient is at risk for substance abuse. 9. Evidence of current physical, sexual, or emotional abuse 10. Living with anyone who currently abuses stimulants or cocaine 11. History of bipolar disorder in both biological parents