Objective: One of the potential causes of residual symptoms of ADHD in adults can be difficulties with consistent adherence to medications. Method: This formative study examined self-reported medication adherence in adults with ADHD with clinically significant symptoms despite medication treatment. Results: Mean adherence for the two-week period prior to the assessment point was 86%, with 18% of the sample reporting less than 80% adherence, and 43% less than 90% adherence. Adherence correlated with ADHD symptoms but not anxiety or depression. Those with less than 80% adherence had higher ADHD severity compared to those whose adherence was at least 80%. Conclusion: These data suggest that self-report of adherence to ADHD medications may be a useful and expedient way of assessing adherence, and that assessment and counseling about adherence may be an important part of treatment. Future research using an objective indicator of adherence is needed to follow up on these findings.