Objective: To compare the results of categorically based versus dimensionally based scoring algorithms for a Diagnostic and Statistical Manual of Mental Disorders (4th ed.; DSM-IV)–referenced teacher rating scale for assessing ADHD and commonly co-occurring conditions and to determine their relative agreement with ratings of symptom-induced impairment. Method: Teachers completed Child and Adolescent Symptom Inventory–4R (CASI-4R) ratings for 1,092 youth (ages 6-18 years) referred to a child and adolescent psychiatry outpatient service. Caseness was determined according to DSM-IV symptom count (categorical model) and T-score (dimensional model) criteria. Results: Agreement between symptom count and T-score cutoffs was generally good (kappa ≥ 0.61) for ADHD-Inattentive, ADHD-Hyperactive-Impulsive, ADHD-Combined (except adolescent females), Oppositional Defiant Disorder, and Conduct Disorder, but this was not the case for anxiety and depressive disorders where only 15% of kappas were good. Agreement of impairment cutoff with T-score and symptom count cutoffs ranged from poor to good. Conclusion: In general, although in many cases CASI-4R categorical and dimensional scoring algorithms generated similar results, there was considerable variability across disorders, age groups, scoring method, and in some cases, gender. Moreover, symptom counts and T-scores are not a proxy for assessing impairment suggesting that each scoring strategy likely provides unique information for clinical decision-making. (J. of Att. Dis. 2013; XX(X) 1-XX)