Depression is not a homogenous illness and is diagnosed in only one in three persons suffering from this disorder. Besides psychological-psychiatric diagnostic methods (e.g. Hamilton's, Beck's rating scales), some biochemical measures have been introduced as markers of depression. However, the sensitivity of even the best characterized dexamethasone suppression test is as low as 40-50%. A search for better markers is continuously in progress. In the present study, we investigated alterations in serum copper concentrations in depressive patients before, during and after antidepressant treatment and compared these values with the concentrations in healthy volunteers. The serum copper concentration in depression is significantly higher (by 21%) than in the control. However, effective antidepressant treatments (which reduced symptoms by ca. 50% measured by Hamilton Depression Rating Scale) did not affect (did not normalize) this increased copper concentration. The present data indicate that serum copper is a "trait marker" (remains constant regardless of successful treatment) of unipolar depression.