This investigation examined how two commonly employed measures of functional impairment (the "Child and Adolescent Functional Assessment Scale" [CAFAS] and the "Global Assessment of Functioning" [GAF] scale) perform with respect to assessing levels of functional impairment and, hence, identifying rates of serious emotional disturbance (SED) across youth diagnosed with internalizing or externalizing disorders. GAF scores indicated comparable levels of functional impairment between groups. CAFAS scores suggested significantly higher levels of functional impairment for youth diagnosed with externalizing disorders. The CAFAS also identified significantly higher rates of SED among youth with externalizing than those with internalizing disorders, whereas the GAF identified comparable rates of SED between groups. These findings suggest caution when relying on a single measure to assess functional impairment and to identify SED, particularly when these measures are used in decisions concerning service allocation. Implications of these findings are addressed, including the need for further research to identify the optimal assessment strategy to assess functional impairment and identify SED among youth diagnosed with internalizing or externalizing disorders. (Contains 1 note and 3 tables.)