AIMS: Attention deficit/hyperactivity disorder (ADHD) is common among adult patients with substance use disorders. The benefits of treating ADHD in these patients are uncertain and the prescription of psychostimulants is disputed, because of the risk of abuse. This study examined the short-term effectiveness of methylphenidate treatment for ADHD in adults with substance use disorders. DESIGN: Double-blind, placebo-controlled, multiple cross-over (A-B-A-B design) comparative trial of methylphenidate versus placebo. SETTING: In-patient addiction treatment facility. PARTICIPANTS: Twenty-five patients with ADHD who were receiving in-patient treatment for various substance use disorders. INTERVENTION: During the course of 8 weeks, each participant completed two phases of placebo and two phases of active medication treatment, in a fixed low-dosage schedule (up to 0.6 mg/kg/day). Abstinence was maintained during the study. MEASUREMENTS: The outcome measure was ADHD symptomatology, as measured with the ADHD rating scale-IV. The results were compared using MANOVA repeated measures. FINDINGS: Nineteen of the 25 patients completed the trial. A significant reduction in ADHD symptoms was observed in the first week in both conditions. The positive response to active treatment (nine patients; 36%) was not significantly higher than that to placebo (five patients; 20%). CONCLUSIONS: In this small pilot study, the effect of low-dose methylphenidate in adult ADHD patients with concomitant substance use disorders is limited. ADHD symptoms in adults were susceptible to a distinct short-term placebo response.