Objective: To compare the rate of persistence of ADHD into adulthood as determined by a norm-referenced versus non-norm-referenced diagnostic interview, and by standardized questionnaires. Method: Adults from a birth cohort, including research-identified childhood ADHD cases (N = 232; M_age = 27.0 years; 167 males, 65 females) and controls (N = 335; M_age = 28.6 years; 210 males, 125 females), were administered the M.I.N.I. International Neuropsychiatric Interview, the Murphy–Barkley Symptoms Checklist (MB), and the Wender Utah Rating Scale (WURS) Results: Among the childhood ADHD cases, 29.3% fulfilled criteria for adult ADHD using a norm-referenced approach to M.I.N.I. scoring, versus 13.8% using published M.I.N.I. criteria. Among participants meeting norm-referenced diagnostic criteria, 41.8% and 69.1% were classified as adult ADHD using the MB and WURS, respectively. Conclusion: A non-norm-referenced approach resulted in a significant underestimate of the rate of adult ADHD. Reliance on either of two adult ADHD questionnaires would have further reduced this estimate.