The rate of manifestation of depressive episodes can vary appreciably. The complete development of a depressive episode may be very rapid, taking less than one hour or be very slow, taking up to one month. Altough this clinical observation suggests different neurobiological pathomechanisms, the onset speed of depressive episodes in different affective disorders has not been investigated systematically up to now. The objective of this study was to establish the onset speed of depressive episodes in patients with a history of at least one depressive episode and to compare the onset speed in unipolar with that in bipolar affective disorders.

A group of 96 inpatients was investigated consecutively using the structured patient interview “Onset of Depression Inventory” (ODI). In 76 patients, there was a unipolar depressive disorder and 20 patients suffered from a bipolar depressive disorder.

The onset speed of the current depressive episode in patients with effective disorders correlated significantly with the onset speed of the previous depressive episodes (p < 0.001). Furthermore, there was a significant difference in the onset of the depressive episodes between unipolar and bipolar affective disorders (p < 0.001). In 55% of patients with a bipolar disorder, the depressive episode was manifested within one week whereas this was the case in only 22,37% of the patients with a unipolar affective disorder.

The rate of manifestation of depressive episodes differs between unipolar and bipolar disorders. The clinical observation reported here can support the diagnostic appraisal of depressive episodes.