BACKGROUND Previous studies have suggested an association between abnormal endothelial function and depression. We therefore tested this hypothesis in patients with depression in the course of mood disorders and assessed the effect of antidepressant treatment. METHODS Psychometric evaluation and hemodynamic and endothelial function studies using pulse wave analysis were performed on a group of 31 patients with unipolar or bipolar depression. The control group consisted of 18 healthy subjects, age- and gender-matched. RESULTS Arterial endothelial function was impaired in patients compared with that in control subjects, both during a depressive episode and when in remission after pharmacological treatment. The diagnosis, intensity of depression, and type of antidepressant drugs did not influence the results. CONCLUSIONS The impairment of endothelial function might constitute a trait marker of the biological make-up of patients with mood disorders and might contribute to the increased frequency of cardiovascular conditions observed in these patients.