Major depressive disorder (MDD) is often considered an episodic disorder. However, literature might underestimate the chronicity of MDD since results depend on follow-up duration and the extent to which psychiatric co-morbidity is taken into account.

To determine, whether MDD should be considered an episodic or chronic disorder.

To examine the 6 year course of MDD, incorporating data of multiple time points and taking common psychiatric comorbidities into account.

Data were from 903 patients with current MDD at baseline in the Netherlands study of depression and anxiety, with subsequent data from 2 year, 4 year and 6 year follow-up. Four course trajectories were created taking all information during follow-up into account classifying patients as (1) recovered, (2) recurrent without chronic episodes, (3) recurrent with chronic episodes or (4) consistently chronic. A chronic episode was defined as having symptoms consistently over 2 years.

The recovery rate of MDD was 58% at 2 year follow-up but looking at 6 year follow-up and taking into account co-morbid dysthymia, (hypo) mania and anxiety disorders reduced this recovery rate to 17%. Moreover, more than half of the patients experienced chronic episodes.

Longitudinal data of this psychiatric cohort study showed that full recovery is the exception rather than the rule. MDD follows a chronic course and, moreover, persons are prone to switch to other psychiatric disorders.