Abstract Background Emotional dysregulation (ED) is now considered as an important symptom of attention deficit hyperactivity disorder (ADHD). It is believed to have a considerable impact on the severity of the disorder, one’s global functioning, and the prognosis. Our research aimed to evaluate and compare ED and cognitive emotional regulation strategies between ADHD and borderline personality disorder (BPD) patients. Methods Four hundred six French-speaking outpatients (N = 279 ADHD, N = 70 BPD, N = 60 BPD + ADHD) were assessed with the Emotion Reactivity Scale (ERS), the Cognitive Emotional Regulation Questionnaire (CERQ), The Basic Empathy Scale (BES-A), the Adult ADHD Self-Report Scale (ARSV-v1.1) and the Beck Depression Inventory II (BDI-II). ADHD, BPD and comorbid patients were compared with each other and with samples of controls extracted from already published data. Results ADHD patients, although having higher ED than samples derived from the general population, had less ED, better control over their emotions with higher use of adaptive cognitive strategies and lesser use of non-adaptive strategies than BPD patients. However, ADHD subjects had similar scores as BPD subjects when looking at difficulties in perceiving self and others. ED generated considerable distress in all groups and was also positively associated with ADHD symptomatology. ADHD patients with comorbid BPD had the highest scores of ED. Conclusions Our results suggest that there may be similarly inefficient cognitive emotional regulation skills leading to ED in both disorders (ADHD and BPD). However, ADHD patients showed a higher use of adaptive cognitive emotional strategies and a lower level of ED than BPD patients.