['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.', '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.', 'There was a significant positive association between speed of onset of the present depressive episode and that of the preceding depressive episode (rho = 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.'] Is onset of depressive episodes faster in patients with bipolar versus unipolar depressive disorder : evidence from a retrospective comparative study?