Attention deficit hyperactivity disorder (ADHD) is among the most common, intensely investigated, and yet diagnostically controversial neurobehavioral conditions of childhood. The prevalence of ADHD has been reported with great variations among different studies, ranging from 2.2% to 17.8%. The aim of this review was to investigate the variables that influence the prevalence of ADHD and to derive a best estimate for the prevalence of the disorder. We reviewed all the 39 studies on ADHD prevalence appearing in the Pubmed and published since 1992. These studies indicate that ADHD is more common in boys than girls, in younger than older children and adolescents, in one-setting rather than two-setting screening studies, in studies based on DSM-IV rather than DSM-III-R criteria. Additional factors that may well influence prevalence rates include source of information and assessment of clinical impairment. In conclusion, our findings suggest that population characteristics, methodology features, ethnic and cultural differences and diagnostic criteria involved in studies affect the prevalence of ADHD. Standardized designs may lead to firm conclusions on the true prevalence of ADHD, the estimation of which seems impossible to be achieved by reviewing the already existing literature.