Guidelines for the treatment of attention-deficit hyperactivity disorder (ADHD) in adults advocate methylphenidate as first-line treatment. The aim of this study was to review the effectiveness of methylphenidate treatment of adult ADHD and to examine the influence of methods on meta-analytic results. Electronic databases were searched to identify clinical trials comparing methylphenidate with placebo in the treatment of adult ADHD. Studies were summarised with meta-analytic methods. Subgroup analyses were conducted with respect to parallel group versus cross-over trials and self versus observer ratings. The relationship between dosage and effect size was explored by weighted regression analysis. The results were tested for publication bias, and several sensitivity analyses were performed. Findings and methods were compared with a previous meta-analysis. Eighteen studies met the inclusion criteria of which 16 were included in the meta-analysis. The overall effect size (d = 0.42) was significantly different from zero, but was only half the size expected on the basis of a previous meta-analysis. No significant differences could be observed in the subgroup analyses. The regression analysis showed no significant influence of mean daily dose on effect size. These results contradict findings of a previous meta-analysis and challenge guideline recommendations. Methodological issues in meta-analyses are discussed.