AbstractObjective:To investigate distinguishing features between bipolar I, II and unipolar depression, and impulsivity/aggression traits in particular.Methods:Six hundred and eighty-five (n = 685) patients in a major depressive episode with lifetime Unipolar (UP) depression (n = 455), Bipolar I (BP-I) disorder (n = 151), and Bipolar II (BP-II) (n = 79) disorder were compared in terms of their socio-demographic and clinical characteristics.Results:Compared to unipolar patients, BP-I and BP-II depressed patients were significantly younger at onset of their first depressive episode, and were more likely to experience their first depressive episode before/at age of 15. They also had more previous affective episodes, more first- and second-degree relatives with history of mania, more current psychotic and subsyndromal manic symptoms, and received psychopharmacological and psychotherapy treatment at an earlier age. Furthermore, BP-I and BP-II depressed patients had higher lifetime impulsivity, aggression, and hostility scores. With regard to bipolar subtypes, BP-I patients had more trait-impulsivity and lifetime aggression than BP-II patients whereas the latter had more hostility than BP-I patients. As for co-morbid disorders, Cluster A and B Personality Disorders, alcohol and substance abuse/dependence and anxiety disorders were more prevalent in BP-I and BP-II than in unipolar patients. Whereas the three groups did not differ on other socio-demographic variables, BP-I patients were significantly more often unemployed that UP patients.Conclusion:Our findings comport with major previous findings on differences between bipolar and unipolar depression. As for trait characteristics, bipolar I and II depressed patients had more life-time impulsivity and aggression/hostility than unipolar patients. In addition, bipolar I and II patients also differed on these trait characteristics.