Background. The association between depression and mortality has become a topic of interest. Little is known about the association between the course of depression and mortality.

Methods. In an initially non-depressed cohort (N = 325) and a depressed cohort (N = 327), depression was measured using the Center for Epidemiologic Studies Depression scale (CES-D) at eight successive waves over a period of 3 years. Both cohorts were then followed with respect to mortality status for up to 3·5 additional years. Clinical course types as well as theoretical course type parameters (basic symptom levels, increases in symptoms and instability over time) were distinguished to study the effect of the course of depression on mortality.

Results. Contrary to transient states of depression, both chronic depression and chronic intermittent depression predicted mortality at follow-up. Additionally, evidence was found that the effect on mortality is related to severity of depression; high basic symptom levels and increases in symptoms over time were predictive of mortality. A high degree of instability over time was not associated with mortality.

Conclusions. Since the mortality effect of depression is a function of both exposure time and symptom severity, more attention should be paid to the treatment of depression in order to prevent severe longstanding depression.