 The hypothesis is that Osmotic Release Oral System OROS Extended Release Methylphenidate HCL OROS MPH is safe and effective in improving Attention Deficit Hyperactivity Disorder ADHD symptoms in adults with ADHD when compared to placebo as demonstrated using specific study measures. This is a double-blind neither participant nor investigator knows the name of the assigned study drug, randomized study drug assigned by chance, placebo-controlled study to evaluate the efficacy and safety of OROS MPH using the optimal dose for each adult patient in the study with ADHD. The primary efficacy variable in this study is the change from baseline to final evaluation using the Adult ADHD Investigator Symptom Rating Scale AISRS to measure patient ADHD symptoms. Participants will also be assessed for adverse events throughout the study. Patients will initiate treatment with oral OROS MPH 18 mg or matching placebo at baseline and continue morning dosing with increases every week until an optimal dose is achieved, up to the maximum of 72 mgday of OROS MPH or matching placebo. Eligible patients will remain in the study for a maximum of 6 weeks. 