Context: Substance use disorders (SUD) are a major public health problem. Attention deficit hyperactivity disorder (ADHD) is a comorbid condition associated with both onset and prognosis of SUD. Prevalence estimates of ADHD in SUD vary significantly. Objective: To obtain a best estimate of the prevalence of ADHD in SUD populations. Data sources: A literature search was conducted using MEDLINE, PsycINFO and EMBASE. Search terms were ADHD, substance-related disorders, addiction, drug abuse, drug dependence, alcohol abuse, alcoholism, comorbidity, and prevalence. Results were limited to the English language. Study selection: After assessing the quality of the retrieved studies, 29 studies were selected. Studies in which nicotine was the primary drug of abuse were not included. Data extraction: All relevant data were extracted and analysed in a meta-analysis. A series of meta-regression analyses was performed to evaluate the effect of age, primary substance of abuse, setting and assessment procedure on the prevalence of ADHD in a variety of SUD populations. Data synthesis: Overall, 23.1% (CI: 19.4-27.2%) of all SUD subjects met DSM-criteria for comorbid ADHD. Cocaine dependence was associated with lower ADHD prevalence than alcohol dependence, opioid dependence and other addictions. Studies using the DICA or the SADS-L for the diagnosis of ADHD showed significantly higher comorbidity rates than studies using the KSADS, DISC, DIS or other assessment instruments. Conclusions: ADHD is present in almost one out of every four patients with SUD. The prevalence estimate is dependent on substance of abuse and assessment instrument. © 2011 Elsevier Ireland Ltd.