Impulsivity is regarded as a key feature in borderline personality disorder (BPD). However, discrepancies in previous research indicate that the role of impulsivity in BPD is not yet fully understood. For example, state-dependent impulsivity in individuals with BPD may be related to co-occurring psychiatric conditions such as attention-deficit/hyperactivity disorder (ADHD) and to emotional states. We assessed self-reports of trait and state impulsivity and response inhibition before and after an experimental stress induction in 15 patients with BPD without ADHD, 15 patients with BPD and ADHD, 15 patients with ADHD, and 15 healthy participants. The patients in both BPD subgroups reported a stress-dependent increase of state impulsivity, which was not observed in the other groups. Response inhibition was impaired in the patients with BPD and ADHD but not in those without ADHD compared with the healthy participants. We suggest that stress levels and co-occurring ADHD should receive attention in future studies on impulsivity in BPD.