Background and Objectives: Emotional dysregulation is central to the problem of the overlap between attention-deficit/hyperactivity disorder (ADHD) and cyclothymia. The aim of the study was to evaluate comorbidity rates between ADHD and cyclothymic disorder and to explore demographic and clinical differences among the groups, focusing on affective temperament and emotional dysregulation. Materials and Methods: One hundred sixty-five outpatients attending the Second Psychiatry Unit at the Santa Chiara University Hospital (Pisa) were consecutively recruited: 80 were diagnosed with ADHD, 60 with cyclothymic disorder, and 25 with both conditions. Temperament Evaluation of Memphis, Pisa, Paris, and San Diego (TEMPS-M) and the 40-item version of Reactivity, Intensity, Polarity, and Stability questionnaire (RI-PoSt-40) were administered. Results: Cyclothymic patients were more frequently female and older with respect to the ADHD groups. Both comorbid and non-comorbid ADHD patients showed significantly lower educational attainment and more frequently had substance use disorders. Panic disorder was common in non-comorbid cyclothymic patients, who showed significantly higher rates of familial panic disorder, major depressive disorder and suicide attempts in comparison with patients only diagnosed with ADHD. Cyclothymic patients without ADHD were also characterized by fewer hyperthymic temperamental traits, higher depressive and anxious dispositions, and a greater negative emotionality. No significant differences among groups were observed for cyclothymic temperament and overall negative emotional dysregulation, but comorbid patients with both conditions scored the highest in these subscales. This group also showed significantly higher affective instability with respect to ADHD patients without cyclothymia and was less frequently diagnosed with bipolar disorder type II than patients from both the other groups. Conclusions: ADHD and cyclothymia often co-occur and show similar levels of emotional dysregulation. However, cyclothymic patients may be more prone to negative emotionality in clinical settings. Subjects with “sunny” cyclothymic features might escape the attention of clinicians unless ADHD is present.