Objective: To evaluate the efficacy and safety of osmotic-release methylphenidate (OROS-MPH) compared with placebo for attention-deficit/hyperactivity disorder (ADHD), and the impact on substance treatment outcomes in adolescents concurrently receiving cognitive-behavioral therapy (CBT) for substance use disorders (SUD). Method: This was a 16-week, randomized, controlled, multi-site trial of OROS-MPH + CBT versus placebo + CBT in 303 adolescents (aged 13 through 18 years) meeting "DSM-IV" diagnostic criteria for ADHD and SUD. Primary outcome measures included the following: for ADHD, clinician-administered ADHD Rating Scale (ADHD-RS), adolescent informant; for substance use, adolescent-reported days of use in the past 28 days. Secondary outcome measures included parent ADHD-RS and weekly urine drug screens (UDS). Results: There were no group differences on reduction in ADHD-RS scores (OROS-MPH: -19.2, 95% confidence interval [CI], -17.1 to -21.2; placebo, -21.2, 95% CI, -19.1 to -23.2) or reduction in days of substance use (OROS-MPH: -5.7 days, 95% CI, 4.0-7.4; placebo: -5.2 days, 95% CI, 3.5-7.0). Some secondary outcomes favored OROS-MPH, including lower parent ADHD-RS scores at 8 (mean difference = 4.4, 95% CI, 0.8-7.9) and 16 weeks (mean difference =6.9; 95% CI, 2.9-10.9) and more negative UDS in OROS-MPH (mean = 3.8) compared with placebo (mean = 2.8; p = 0.04). Conclusions: OROS-MPH did not show greater efficacy than placebo for ADHD or on reduction in substance use in adolescents concurrently receiving individual CBT for co-occurring SUD. However, OROS-MPH was relatively well tolerated and was associated with modestly greater clinical improvement on some secondary ADHD and substance outcome measures. Clinical Trial Registration Information-Attention Deficit Hyperactivity Disorder (ADHD) in Adolescents with Substance Use Disorders (SUD); NCT00264797. (Contains 3 figures and 2 tables.)