A significant comorbidity between attention-deficit/hyperactivity disorder (ADHD) and affective disorders has been consistently reported in adults. Less data regarding the role of personality traits and the influence of ADHD co-occurrence on clinical characteristics and outcome of mood disorders are currently available. One hundred and six remitted major depressed, 102 euthymic bipolar subjects, and 120 healthy controls, homogeneous with respect to demographic characteristics, were included in the study. ADHD diagnosis was based on DSM-IV-TR criteria. Childhood and adult ADHD features were measured with the Wender Utah Rating Scale, the Adult ADHD Self-rating Scale, and the Brown Attention-Deficit Disorder Scale. The Revised NEO Personality Inventory was also administered to the clinical groups, in order to investigate personality dimensions. The occurrence of adult ADHD in subjects with bipolar disorders (BD) or major depressive disorder (MDD) was 15.7 and 7.5 %, respectively, compared to 3.3 % in healthy controls (HC). Significant associations (p < .001) between personality traits (neuroticism, conscientiousness, and extraversion) and ADHD features were observed. Logistic regression analysis of all clinical subjects (n = 208) showed that those with lower levels of neuroticism (OR = 1.031; p = .025) had a lower frequency of ADHD comorbidity. The present study emphasizes the close relationship between affective disorders, especially BD, and ADHD in adults. Our findings support the need to assess subjects with mood disorders in the clinical setting for possible coexisting ADHD and to further investigate personality traits to better understand the etiology of affective disorders and ADHD co-occurrence.