A significant proportion of adults diagnosed with ADHD also exhibit emotional dysregulation (ED) in addition to the core symptoms of the condition. However, ADHD-associated ED is neither consistently conceptualised nor taken into account diagnostically. Furthermore, the distinction between patients with ADHD who do and do not also exhibit ED remains ambiguous. Research has demonstrated that individuals diagnosed with ADHD exhibit unstable EEG-based regulation of their brain arousal. In contrast, individuals diagnosed with depression exhibit stable brain arousal regulation in the EEG. In light of these findings, we have formulated a hypothesis that an additional ED may influence brain arousal regulation in individuals with ADHD, thereby resulting in measurable differences in the EEG. EEG data were collected at a specialized outpatient clinic for adult ADHD of our department. Data were analyzed retrospectively by an EEG-based algorithm (VIGALL 2.1) assessing brain arousal. There was no difference in brain arousal between ADHD subgroups with or without ED. A more severe symptomatology in the ADHD with ED subgroup was observed. ED is a common symptomatic domain in ADHD contributing to a more severe symptomatology. The identification of a consensus on the concept of ED and the development of an operationalisation based on specific questionnaires could facilitate the establishment of a foundation for further studies. The brain arousal regulation model provides insights into the aetiology of ED, paving the way for further investigations into clinical subgrouping and targeted treatments.