BACKGROUND In the last decades affective disorders were divided into unipolar and bipolar and this division has been generally accepted. The bipolar type is manifested by mania or by both mania and depression. On the other hand, unipolar affective disorders are manifested only by depression. In numerous investigations authors have noticed that there are very distinctive differences between these two types of depressive disorders such as: course of illness, personality disorders, sex, family history etc. Nevertheless, in practice it is often very difficult to make the right diagnosis. The bipolar type often starts with a few pure depressive episodes and sometimes mania occurs a few years later so only at that point the psychiatrist can make the right diagnosis and treat the patient correctly. METHODS This investigation comprised 50 patients hospitalized at the Psychiatric Clinic in Novi Sad during 1992-1995. The experimental group consisted of 20 patients with a bipolar affective disorder (according to ICD-X), while the control group consisted of 30 patients with clinical diagnosis of unipolar depression (intensive, without psychiatric features). Both groups of patients were weekly evaluated by Hamilton Depression Rating Scale (HDRS), whereas the initial score for all patients had to be higher than 16. RESULTS Patients suffering from unipolar depression were older than patients with bipolar depression and there were more females in this group. There were no differences in demographic characteristics (level of education, migration, etc.), but the experimental group had a greater genetic loading for affective disorders. Unipolar depressive patients had more agitation and they were more anxious than patients with bipolar depression. CONCLUSIONS The fact that unipolar depressive patients were older than bipolar is similar to most of the results gained in this kind of investigation. On the other hand, we did not find statistical differences in the intensity of disorders, and in the literature these results are contraindicating. Numerous investigators report that bipolar depressives had a stronger genetic loading for affective disorders and our study confirms the same. All these results can help us to make the right diagnosis of unipolar and bipolar affective disorders.