Background: Patients with attention-deficit/hyperactivity disorder (ADHD) and patients with borderline personality disorder (BPD) show symptomatic overlaps. They both suffer from deficits in emotional regulation, are impulsive and have problems with their self-concept. Therefore, a precise diagnostic differentiation is of great importance. The aim of this study was to find symptom overlaps and differences in patients with ADHD and BPD. Methods: 80 patients with ADHD, 55 patients with BPD and 55 healthy controls were examined regarding their ADHD and BPD symptoms and their degree of emotional dysregulation using self-report instruments. Results: Patients with ADHD and patients with BPD did not differ significantly in their expression of emotional dysregulation. However, the ADHD patients showed higher scores in impulsivity, inattention, and hyperactivity, whereas the group with BPD showed higher scores in self-concept problems and suicidal behaviour. The two clinical groups showed significantly higher scores in emotional dysregulation and all other symptom domains compared to the control group. Conclusion: The symptom overlap in emotional dysregulation yields implications for both further research and diagnosis of ADHD. Further studies should define emotional dysregulation consistently in order to examine the same construct.