Objective Longitudinal studies of children diagnosed with ADHD report widely ranging ADHD persistence rates in adulthood (5–75%). This study documents how information source (parent vs. self‐report), method (rating scale vs. interview), and symptom threshold (DSM vs. norm‐based) influence reported ADHD persistence rates in adulthood. Method Five hundred seventy‐nine children were diagnosed with DSM‐IV ADHD‐Combined Type at baseline (ages 7.0–9.9 years) 289 classmates served as a local normative comparison group (LNCG), 476 and 241 of whom respectively were evaluated in adulthood (Mean Age = 24.7). Parent and self‐reports of symptoms and impairment on rating scales and structured interviews were used to investigate ADHD persistence in adulthood. Results Persistence rates were higher when using parent rather than self‐reports, structured interviews rather than rating scales (for self‐report but not parent report), and a norm‐based (NB) threshold of 4 symptoms rather than DSM criteria. Receiver‐Operating Characteristics (ROC) analyses revealed that sensitivity and specificity were optimized by combining parent and self‐reports on a rating scale and applying a NB threshold. Conclusion The interview format optimizes young adult self‐reporting when parent reports are not available. However, the combination of parent and self‐reports from rating scales, using an ‘or’ rule and a NB threshold optimized the balance between sensitivity and specificity. With this definition, 60% of the ADHD group demonstrated symptom persistence and 41% met both symptom and impairment criteria in adulthood.