Interferon-alpha (IFN-alpha) treatment is frequently complicated by symptoms of depression. The mechanism by which peripherally administered IFN-alpha enters and modulates the central nervous system remains unclear. The cell adhesion molecule ICAM-1 is involved in the regulation of blood-brain barrier (BBB) permeability. ICAM-1 expression was shown to increase during IFN-alpha treatment and recently the expression of ICAM-1 on vascular endothelial cells in the brain was found to be correlated with the development of depression. We therefore hypothesized that soluble ICAM-1 may be involved in the development of IFN-alpha associated depression. In a prospective study, serum levels of soluble ICAM-1 (double sandwich ELISA test) and symptoms of depression (SDS) were measured in 48 patients with malignant melanoma before and during adjuvant IFN-alpha treatment. Both, depression scores and the serum levels of sICAM-1 significantly increased after three months of IFN-alpha treatment compared to baseline levels (p < .001). Patients who developed depression (SDS-index scores > or = 50) after three months of treatment had higher sICAM-1 levels compared to non-depressed patients. Furthermore, sICAM-1 levels were positively correlated with SDS values (r = .367, p = .018). Our data provides evidence for an association between the induction of sICAM-1 and the development of symptoms of depression during IFN-alpha treatment, possibly by enhancing BBB-permeability.