Cognitive reactivity to sad mood refers to the degree to which a mild dysphoric state reactivates negative thinking patterns. In this research, the contribution of the history of depression, the length of the current depressive episode and the intensity of the depressive symptoms were assessed in explaining the cognitive reactivity to sad mood measured with the Leiden Index of Depression Sensitivity (LEIDS). The sample consisted of 123 depressed outpatients. The results of principal components analysis suggested a three-factor solution of the LEIDS. The intensity of depressive symptoms, the history of depression and the length of the current depressive episode were all significant in explaining cognitive reactivity to sad mood. We have also found out a significant effect of interaction of the history of depression and the length of the current depressive episode, which demonstrated that a prolonged depression does not induce a stronger cognitive reactivity to sad mood during the relapse of a depressive episode, while during the first depressive episode a longer duration of depression does induce a stronger cognitive reactivity. Such a result demonstrates that the length of the first depressive episode, regardless of its intensity, is crucially important for the formation of cognitive reactivity.