Objective: ADHD is a common comorbid condition with substance use disorder. This study seeks to examine the discrepancy in the prevalence rate between those previously diagnosed with ADHD and those diagnosed while in treatment. It is hypothesized that clients with ADHD would have earlier unsuccessful terminations from treatment than non-ADHD clients and that the ADHD Self-Report Scale (ASRS Version 1.1) would be a reliable predictor. Method: Participants (N = 87) are admitted to a publicly funded 28-day residential treatment program. All participants are screened with the ASRS and participate in a clinical assessment evaluation. Results: A significant difference is found between the clinical record rate of 3.44% and the 43.68% rate found during treatment. The ASRS significantly predicts ADHD. Conclusions: The use of the ASRS is recommended and should be incorporated into standard intake assessment protocols. Careful diagnostic interviews are urged to determine if clients in residential treatment have ADHD.