In spite of the growing literature about adult attention-deficit hyperactivity disorder (ADHD), relatively little is known about the prevalence and correlates of this disorder. To estimate the prevalence of adult ADHD and to identify its demographic correlates using meta-regression analysis. We used the MEDLINE, PsycLit and EMBASE databases as well as hand-searching to find relevant publications. The pooled prevalence of adult ADHD was 2.5% (95% CI 2.1-3.1). Gender and mean age, interacting with each other, were significantly related to prevalence of ADHD. Meta-regression analysis indicated that the proportion of participants with ADHD decreased with age when men and women were equally represented in the sample. Prevalence of ADHD in adults declines with age in the general population. We think, however, that the unclear validity of DSM-IV diagnostic criteria for this condition can lead to reduced prevalence rates by underestimation of the prevalence of adult ADHD.