ABSTRACT: BACKGROUND: The aims of this study were to assess the prevalence of a comorbid Attention Deficit Hyperactivity Disorder (ADHD) diagnosis in Borderline Personality Disorder (BPD), and its impact on the clinical presentation of BPD in adolescents, and to determine which type of impulsivity specifically characterizes adolescents with BPD-ADHD. METHODS: ADHD diagnoses were sought in a sample of 85 DSM-IV BPD adolescents drawn from the EURNET BPD. Axis-I and -II disorders were determined with the K-SADS-PL and the SIDP-IV, respectively. Impulsivity was assessed with the BIS-11. RESULTS: 11% (N=9) of BPD participants had a current ADHD diagnosis. BPD-ADHD adolescents showed higher prevalence of ODD and a non-significant trend for a higher prevalence of other cluster B personality disorders. Regression analyses revealed a significant association between cognitive impulsivity scores and ADHD (Wald Z=6.69; p=0.01 ; Exp(B)= 2.02, CI 95% 1.19-3.45). CONCLUSIONS: Comorbid ADHD influences the clinical presentation of adolescents with BPD and is associated with higher rates of ODD, other cluster B personality disorders, and cognitive impulsiveness. A subgroup of BPD patients may exhibit developmentally driven impairments of the inhibitory system persisting since childhood. Specific interventions should be recommended for this subsample of BPD adolescents.