Objective: The Das–Naglieri Cognitive Assessment System (CAS) is a test of cognitive abilities based on the Planning, Attention, Simultaneous, and Successive Theory (PASS). Studies of CAS performance by children with ADHD typically show lowest performance on Planning and deficits on Attention, but normal Simultaneous and Successive processing. Such distinct group differences studies support construct validity and are necessary, but not sufficient, for establishing diagnostic utility. Method: Students meeting Diagnostic and Statistical Manual of Mental Disorders (4th ed., text rev.; DSM-IV-TR) criteria for ADHD (n = 20) and matched controls (n = 20) were assessed with the CAS to examine distinct group differences and diagnostic utility of CAS in correctly classifying the individuals from both groups. Results: CAS PASS scores were significantly related to ADHD characteristics demonstrating both distinct group differences (with medium to large effect sizes) and diagnostic utility (with medium effect sizes). Conclusion: Support was observed for CAS PASS score characteristics previously observed among students with ADHD, and this is the first study to report on the diagnostic utility of CAS PASS scores. Given the small sample, additional large-scale studies and cross-validation is needed. (J. of Att. Dis. 2013; XX(X) 1-XX)