Clinical and research descriptions of depressive symptomatology have generally taken a cross-sectional approach. However, the few existing studies examining longitudinal patterns of onset have found rapidity of onset related to patient/family history of affective disorder, depressive stressors, and treatment response. Patients with an onset of major depressive disorder during the previous 5-56 weeks were studied, and information was obtained on which symptoms were present and in which week they began. A rapid onset of symptoms was associated with the endogenous subtype, the absence of past or current nonaffective disorders, older age of first depressive episode, less stressful events before the current episode, and shorter hospitalization. These findings suggest a relationship between rapidity of symptom onset and the concepts of endogenous and primary depression.