Although attention has been given to the intellectual functioning of children with attention-deficit/hyperactivity disorder (ADHD) relative to their non-ADHD peers, few studies have examined intellectual functioning in adults with ADHD. The purpose of the current investigation was to examine differences in intellectual ability between adults with and without ADHD via a meta-analytic review. Of the 33 studies meeting inclusion criteria, primary analyses focused on 18 studies representing 1,031 adults with ADHD and 928 non-ADHD, nonclinical comparison adults and containing Wechsler Adult Intelligence Scale (WAIS; D. Wechsler, 1955, 1981, 1994) Full Scale IQ scores or estimates. A significant effect was found such that adults with ADHD scored lower than non-ADHD adults on WAIS intelligence tests. However, this difference was small and not clinically meaningful. The presence of several moderators reflecting characteristics of the ADHD samples and study methodology suggested that only a subset of adults with ADHD (e.g., those with comorbid disorders) may experience lower general intellectual ability relative to non-ADHD comparison adults. Implications of the findings for clinical and research settings are presented as well as suggestions for improving methodology and reporting in future research.