The main aim of this study was to examine the age-dependent persistence of attention-deficit hyperactivity disorder (ADHD) in boys transitioning from adolescence into early adulthood attending to different definitions of persistence. We conducted a 10-year follow-up study (mean follow-up time=11 years) of 110 boys with ADHD and 105 non-ADHD controls. Both groups were 6-17 years of age at ascertainment. ADHD was considered persistent at follow-up if subjects met full or subthreshold (more than half of the symptoms required for a full diagnosis) Diagnostic and Statistical Manual of Mental Disorders, fourth edition, (DSM-IV) diagnostic criteria, failed to attain functional remission (Global Assessment of Functioning, GAF score < or =60) or were receiving treatment for ADHD. While 65% of children with ADHD no longer met full DSM-IV criteria for ADHD at the 10-year follow-up, 78% of subjects met at least one of our definitions of persistence. Persistence as described above was associated with more psychiatric co-morbidity, more familiality with mood disorders and higher levels of educational and interpersonal impairments than controls. This 10-year longitudinal follow-up study shows that the majority of ADHD boys experience persistent symptoms and functional impairments into early adulthood. Persistence of ADHD is associated with greater psychiatric comorbidity, familiality and functional impairments.