Objective: Depressive episodes can have a very fast onset (< 1 hour) or start very slowly (> 1 month). This interesting aspect, pointing to different neurophysiological pathomechanisms, has not been systematically evaluated so far. The aim of this study was to describe speed of onset of depressive episodes in a consecutive sample of patients with at least 1 depressive episode and to investigate potential differences between patients with major depression versus bipolar affective disorders concerning this variable.

Method: We examined 158 consecutive adult patients with major depression (N = 108) and bipolar disorder (N = 50) diagnosed according to criteria of the International Statistical Classification of Diseases, 10th revision, by applying the structured Onset-of-Depression Inventory. Patients with acute critical life events preceding the onset were excluded from final analyses. Data were collected between December 2001 and January 2007.

Results: There was a significant positive association between speed of onset of the present depressive episode and that of the preceding depressive episode (r = 0.66, p < .001). Patients with bipolar disorder developed full-blown depressive episodes significantly faster than patients with major depression (p < .001): Whereas depressive episodes began within 1 week in 58% of patients with bipolar disorder, this was the case in only 7.4% of patients with major depression.

Conclusion: Intraindividually, the speed of onset of depression is similar across different episodes. In the absence of acute critical life events, fast onset of depressive episodes (within 1 week) is common in bipolar disorder but rare in major depression. This aspect might be useful to identify depressive episodes occurring within a bipolar affective illness and might characterize a subgroup of patients with a distinct neurobiology.