The purpose of this study is to evaluate the effectiveness and safety of three doses of prolonged-release methylphenidate (a central nervous system (CNS) stimulant) in adult patients with attention deficit/hyperactivity disorder (ADHD).

Stimulant therapy is the mainstay of pharmacologic treatment for attention deficit/hyperactivity disorder (ADHD) in children and adolescents. Methylphenidate is the most commonly prescribed and most frequently studied stimulant medication for children and adolescents with this disorder. It is widely acknowledged in the scientific literature that ADHD often will persist into adulthood. Few large-scale, rigorous studies have been conducted to investigate the effectiveness and safety of stimulant therapy in treating ADHD in adults. This is a multicentre, double-blind, randomised, placebo-controlled, parallel group, dose-response study comparing three doses of prolonged-release methylphenidate (18, 36, and 72 mg once-daily) versus placebo in adult patients with ADHD. The primary efficacy outcome will be the change from baseline to the end of the double-blind phase in the sum of the inattention and hyperactivity/impulsivity subscale scores of the investigator-rated Conners' Adult ADHD Rating Scale (CAARS). Additional measures of effectiveness will include the following scales: CAARS-self report, to rate items pertaining to the patient's behavior/problems; Clinical Global Impression (CGI), to rate the severity of a patient's illness; Sheehan's Disability Scale (SDS), to measure the extent to which a patient's work, social life or leisure activities and home life or family responsibilities are impaired by his/her symptoms; Quality of Life Enjoyment and Satisfaction Questionnaire (Q-LES-Q); and the Global Assessment of Effectiveness. Safety and tolerability will be monitored throughout the study. The study hypothesis is that prolonged-release methylphenidate taken once-daily will produce clinically significant improvements compared with placebo on measures of inattention and hyperactivity/impulsivity of the Conners' Adult ADHD Rating Scale in patients with adult ADHD. Prolonged-release methylphenidate 18, 36 or 72 mg oral capsules, or placebo, taken once-daily for 5 weeks. Eligible patients may continue prolonged-release methylphenidate in an open-label extension study for an additional 7 weeks.

Inclusion Criteria:

• Adults with a diagnosis of Attention Deficit/Hyperactivity Disorder (ADHD) according to DSM-IV criteria
• ADHD symptoms from childhood to adulthood, with some symptoms present before age 7 years which continue to meet DSM-IV criteria at the time of assessment. (ADHD is not diagnosed if the symptoms are better accounted for by another psychiatric disorder.)

Exclusion Criteria:

• Patients known to be a non-responder to methylphenidate or known to have a child who is a non-responder to methylphenidate
• treated with any methylphenidate-containing medication within 1 month of starting the study
• have any clinically unstable psychiatric condition including, but not limited to the following: acute mood disorder, bipolar disorder, acute obsessive-compulsive disorder, anti-social personality disorder, borderline personality disorder
• family history of schizophrenia or affective psychosis
• patients with motor tics or a history of Tourette's syndrome, or with a substance abuse or dependence disorder, an eating disorder, or mental retardation
• using any medications which may affect the evaluation of study results