The primary objective of this study is to evaluate the efficacy of 2 fixed dosages of Prolonged Release (PR) OROS methylphenidate (54 and 72 mg/day) compared with placebo in adult patients with attention deficit/hyperactivity disorder (ADHD). The primary efficacy criterion will be the change in the sum of the inattention and hyperactivity/impulsivity subscale scores of the investigator-rated Conners' Adult ADHD Rating Scale (CAARS), from start of treatment to the end of the double-blind treatment. Hypothesis of the primary objective states that either PR OROS methylphenidate dose is more effective than placebo after 13 weeks of treatment in adult patients with ADHD. This is a multicentre, double-blind, randomized, placebo-controlled, parallel group, dose-response study. Patients will be randomized into one of 3 treatment groups to receive oral dosages of PR OROS methylphenidate 54 or 72 mg, or placebo once daily. The study includes a treatment period of 13 weeks. Patients will be titrated from a starting dose of 36 mg/day for 7 days, to 54 or 72 mg/day at Day 8, after which treatment will be administered for 12 weeks. The study will include a screening period of up to 2 weeks, during which current therapy, not allowed during the study can be tapered down and discontinued (with the exception of fluoxetine or MAO (monoamine oxidase) inhibitors for which a maximum screening period of 4 weeks will be allowed). A post-study visit for collection of additional efficacy and safety data will be scheduled for one week after a patient's final dose of study drug. Adults with a diagnosis of ADHD according to DSM-IV criteria with some symptoms before age 7 years that continue to meet these criteria at the time of assessment will be enrolled in this study. ADHD is not diagnosed if the symptoms are better accounted for by another psychiatric disorder (e.g. mood disorder, anxiety disorder, psychotic disorder, personality disorder). The patient (and if possible, informant) must also describe a chronic course of ADHD symptomatology from childhood to adulthood. The description of this chronic course can be patient-based and previous documented diagnosis and/or treatment is not required. Approximately 300 patients (100 in each randomized treatment group) will participate in this study. The primary efficacy criterion will be the change in the sum of the inattention and hyperactivity/impulsivity subscale scores of the investigator-rated CAARS from baseline to the end of treatment (end of 13 weeks or last post-baseline assessment). This variable will be compared between each dosage group and placebo using an ANCOVA model. Other end points will include changes from baseline to the end of the treatment in the CAARS subscales and assessment of the clinical global impression - global change subscale (CGI-C). Onset of therapeutic effect and responder rate will also be determined. In addition, morning / evening (8 pm) CAARS-S:S assessments will be performed at baseline and on Days 34, 35, 90 and 91. Safety evaluations will include monitoring of adverse events (AEs), clinical laboratory tests (hematology; biochemistry), pregnancy testing, vital signs (supine and standing blood pressure, pulse) and weight, ECG. Patients will be randomized to receive PR OROS methylphenidate 54 or 72 mg, or placebo once daily. Study drug will be administered daily in the morning for up to 13 weeks. Since there is no food effect with methylphenidate, drug administration can be under fed or fasted conditions. Patients will start at a dosage of 36 mg. At Week 2, patients will receive 54 or 72 mg PR OROS methylphenidate for 12 weeks.