Besides the monoamine hypothesis, the stress hypothesis and the vascular hypothesis, the inflammatory hypothesis might be an etiological explanation for late-life depression. There is a growing amount of evidence to support this hypothesis. In animal studies, injection with cytokines was shown to cause behavioural changes ('sickness behaviour') similar to depressive symptoms in humans. Cytokine treatment of certain tumours and chronic hepatitis can also cause depressive symptoms. The prevalence of depression in patients with autoimmune diseases is higher than in the general population. Etanercept had a favourable effect on the depressive symptoms in patients with psoriasis, independent of improvement of physical symptoms. Cytokines affect the hypothalamus-pituitary-adrenal axis and cerebral neurotransmitter systems, both of which are thought to be involved in depression. Immune activation has been associated with depression, and several anti-depressive treatments affect immune parameters, although inconsistently. Since the aging process is associated with a dysregulation of the immune system, the inflammation hypothesis might be particularly true in late-life depression.