This article reviews the efficacy of methylphenidate (MPH) for adult attention-deficit/hyperactivity disorder (ADHD). A literature search identified double-blind placebo-controlled MPH treatment studies of ADHD adults. Meta-analysis estimated the pooled effect size for MPH treatment and tested for publication bias. Meta-analysis regression assessed the influence of study design features on medication effects. Six trials met criteria and were included in this meta-analysis. These studies included a total of 140 MPH-treated ADHD adults and 113 placebo-treated ADHD adults. The mean effect size of 0.9 was statistically significant and there was no evidence of publication bias. Larger MPH effect sizes were associated with physician ratings of outcome and use of higher doses. When treatment is optimized to high doses, the effect size for MPH in adults was 1.3. We found strong support for the assertion that MPH is efficacious for treating adult ADHD. Because the degree of efficacy of MPH in treating ADHD adults is similar to what has been reported from meta-analyses of the child and adolescent literature, our work provides further assurance to clinicians that the diagnosis of ADHD can be validly applied in adulthood.