• The change from Baseline in the Adult ADHD Investigator Symptom Rating Scale (AISRS) total score as assessed by the Investigator at the Final Visit/2 Week Efficacy Assesment Visit
  

• Change from baseline in the AISRS as completed by the Investigator at each Titration Visit. Change from baseline in the AISRS as completed by the Investigator at the end of the study or the last score provided during the study by mg/kg dosing groups.
  

This is a randomized (patients are assigned different treatments based on chance), placebo-controlled , double-blind (neither the patient nor the physician knows whether drug or placebo is being taken), parallel-group, dose-titration, multicenter study to determine the effectiveness and safety of five doses of methylphenidate HCl extended-release tablets, 36 mg, 54 mg, 72 mg (two 36 mg tablets), 90 mg (one 36 mg tablet plus one 54 mg tablet), or 108 mg (two 54 mg tablets) administered orally once per day in adults with Attention Deficit Hyperactivity Disorder. Patients will be randomized to receive either placebo or methylphenidate HCl extended-release tablets for seven weeks. Patients assigned to methylphenidate HCl extended-release tablets will start treatment with 36 mg and continue to receive incremental increases of 18 mg of methylphenidate HCl extended-release tablets every seven days (+/-2 days) until an individualized dose is achieved. The individualized dose is achieved when Adult ADHD Investigator Symptom Rating Scale (AISRS) decreases by 30% and a Clinical Global Impression - Improvement (CGI-I) score is 1 or 2, or titration to the maximum dose of 108 mg is achieved. If a limiting adverse event occurs, the dose will be titrated downward by 18 mg. This dose is then the patient's individualized dose. Once an individualized dose has been achieved, patients will remain on that dose for the duration of the titration period and for the two weeks prior to Final Visit/2-Week Efficacy Assessment Visit. Patients assigned to placebo will follow the same dosing schedule and procedures as the patients randomized to methylphenidate HCl tablets. Safety assessments include monitoring adverse events, blood pressure, pulse, weight, and electrocardiograms (ECG) throughout the study. The primary hypothesis is that methylphenidate HCL extended-release tablets at an individualized dose will be superior to placebo with respect to improvement in the primary efficacy endpoint defined bychange from baseline in the Adult ADHD Investigator Symptom Rating Scale.

Patients will initiate methylphenidate HCl extended-release tablets with 36 mg and continue to receive increases in 18 mg increments for 7 days until an individualized dose or maximum dose of 108 mg is achieved. Patients will remain on this dose for the duration of the titration period and for 2 weeks prior to Final Visit/2 Week Efficacy Assessment; a maximum of 51 days. Patients receiving placebo will follow the same dosing schedule/procedures as patients receiving methylphenidate HCL tablets