This study aimed to estimate the prevalence and correlates of adult attention-deficit hyperactivity disorder (ADHD) in the World Health Organization World Mental Health Survey Initiative. The survey included 11,422 respondents aged 18-44 years from ten countries in the Americas, Europe, and the Middle East. Masked clinical reappraisal interviews were conducted with 154 US respondents to calibrate the screening tool. The estimated prevalence of DSM-IV adult ADHD was 3.4% (range 1.2-7.3%), with lower prevalence in lower-income countries (1.9%) compared to higher-income countries (4.2%). Adult ADHD often co-occurs with other DSM-IV disorders and is associated with significant role disability. Treatment for ADHD is limited, but many cases are treated for comorbid disorders. The study highlights the need for more serious consideration of adult ADHD in future epidemiological and clinical research.This study aimed to estimate the prevalence and correlates of adult attention-deficit hyperactivity disorder (ADHD) in the World Health Organization World Mental Health Survey Initiative. The survey included 11,422 respondents aged 18-44 years from ten countries in the Americas, Europe, and the Middle East. Masked clinical reappraisal interviews were conducted with 154 US respondents to calibrate the screening tool. The estimated prevalence of DSM-IV adult ADHD was 3.4% (range 1.2-7.3%), with lower prevalence in lower-income countries (1.9%) compared to higher-income countries (4.2%). Adult ADHD often co-occurs with other DSM-IV disorders and is associated with significant role disability. Treatment for ADHD is limited, but many cases are treated for comorbid disorders. The study highlights the need for more serious consideration of adult ADHD in future epidemiological and clinical research.