Objective: In adolescents with substance use disorder (SUD), comorbid attention-deficit/hyperactivity disorder (ADHD) is associated with greater severity of substance abuse, conduct problems, and worse treatment outcomes. Although many controlled trials have established the efficacy of psychostimulants, including pemoline, for ADHD in children and adolescents, none have been conducted in adolescents with SUD. This randomized, placebo-controlled trial, conducted between 1996 and 2000, evaluated the safety and efficacy of pemoline on substance abuse and conduct problems. Method: Sixty-nine adolescents (aged 13-19) with conduct disorder (CD), SUD, and ADHD were recruited from the community and randomly assigned to a 12-week clinical trial of pemoline (n = 35) or placebo (n = 34), titrated over 4 weeks to a single morning dose of 75 to 112.5 mg as tolerated. Results: Pemoline had greater efficacy than placebo for ADHD as determined by significantly more Clinician's Global Impression-Improvement (CGI-I) ratings of 1 (very much improved) or 2 (much improved) at the study endpoint (n = 69; p < .05). There was also greater reduction in ADHD severity on the parent-rated Conners Hyperactivity-Impulsivity scale in pemoline-treated study completers compared to placebo-treated completers (pemoline, n = 17; placebo, n = 16; p < .01), but no difference between groups in the intent-to-treat analysis (n = 68; p < .13). Substance use did not decline in either group, and there was no difference between groups in baseline to study endpoint change in substance use or CD symptoms. Overall, pemoline was well tolerated, demonstrating a good safety profile and no elevation in liver enzyme levels. Conclusions: Pemoline was efficacious for ADHD but did not have an impact on CD or substance abuse in the absence of specific treatment for SUD.