BPD is a serious mental illness characterized by emotional dysregulation, the hallmark sign being the presence of a manic mood state. The current study focuses on the largely overlapping symptom pictures and high comorbidity of BPD and attention deficit-hyperactivity disorder (ADHD) in affected youth. Along with the diagnostic similarities, both BPD and ADHD are associated with significant cognitive impairments in executive functioning (EF), the regulatory processes governing planning, problem solving and goal pursuit (Arnsten & Castellanos, 2003; Strakowski, DelBello, Adler, Cecil, & Sax, 2000). Preliminary data suggest that EF deficits may differ between BPD, ADHD and comorbid BPD+ADHD. The goal of the current study was to compare the EF performances of youth diagnosed with ADHD and comorbid BPD+ADHD to that of healthy volunteers on both classic neuropsychological tests and the more functional Behavior Rating Inventory of Executive Functions (BRIEF) parent rating scale. Results showed that, in general, youth diagnosed with BPD+ADHD are more severely impaired than youth diagnosed with ADHD alone and healthy volunteers on measures of EF. These results support the hypothesis that EF deficits independently associated with both BPD and ADHD are compounded in the comorbid condition, as well as the contention that BPD with ADHD is distinct fromADHD alone. Results also indicated that the BRIEF may measure different aspects of EF than the traditional neuropsychological measures. Implications of the EF deficits and their effects on daily life functioning in affected youth are discussed, as is the need to incorporate multiple levels of assessment when evaluating EF abilities.