Introduction: The co-occurrence of attention deficit hyperactivity disorder (ADHD) in affective disorder patients is considerably high. The aims of the present study were to search for the frequency and impact of ADHD co-occurrence on the clinical features of affective disorders and to examine the relationship between the dominant affective temperaments and ADHD. Methods: In total, 100 patients with bipolar disorder (BD), 100 patients with major depressive disorder (MDD), and 100 healthy controls (HC) were included. All diagnoses were assigned according to DSM-IV-TR criteria. The Adult Attention Deficit and Hyperactivity Self-Report Scale (ASRS); Wender Utah Rating Scale (WURS); and Temperament Evaluation of Memphis, Pisa, Paris, and San Diego Autoquestionnaire (TEMPS-A) were applied to all participants. Results: The percentage of BD patients meeting the criteria for a diagnosis of current ADHD was 48% compared with the percentage of MDD patients and HCC subjects, i.e., 25% and 12%, respectively. ADHD was significantly more frequent in bipolar adults than in not only HC but also depressive patients. In the BD group, patients with a comorbid ADHD diagnosis had significantly more suicidal history than those without ADHD. The scores of the temperament traits, namely depressive, cyclothymic, irritable, and anxious, were significantly higher in subjects with ADHD in all groups, including in HC. Conclusion: The most important findings of the present study were the observations that (1) the frequency of ADHD is considerably high among bipolar patients; (2) the frequency of suicide attempts is high in the bipolar patient group with comorbid ADHD; and (3) depressive, cyclothymic, irritable, and anxious temperaments are significantly associated with ADHD comorbidity in bipolar and depressive patients as well as in HC. The high comorbidity and chronic course of ADHD and its possible negative influence on the course of both disorders increase the importance of screening for adult ADHD.