BACKGROUND: Little is known about the epidemiology of adult attention-deficit hyperactivity disorder (ADHD). AIMS: To estimate the prevalence and correlates of DSM-IV Adult ADHD in the World Health Organization World Mental Health Survey Initiative. METHOD: An ADHD screen was administered to respondents aged 18-44 years in ten Countries in the Americas, Europe And the Middle East (n=11422). Masked clinical reappraisal interviews were administered to 154 US respondents to calibrate the screen. Multiple imputation was used to estimate prevalence and correlates based on the assumption of Cross-national calibration comparability. Results: Estimates of ADHD prevalence averaged 3.4% (range 1.2-7.3%), with lower prevalence in lower-income Countries (1.9%) compared with higher-income countries (4.2%). Adult ADHD often co-occurs with other DSM-IV Disorders And is associated with considerable role Disability. Few cases are treated for ADHD, but in many cases treatment is given for comorbid disorders. CONCLUSIONS: Adult ADHD should be considered more seriously in future epidemiological and clinical studies than is currently the case.