Borderline personality disorder (BPD) is typically characterized by instability and impairmed behaviour, affectivity, interpersonal relations and lifestyle. The most common condition comorbid with BPD is a depressive episode. Depression is associated with severe disturbance of the circadian rhythms. This is apparent in depressive patients with BPD. Bright light may be an effective in treatment of seasonal affective disorder. Several studies have suggested antidepressant effects of phototherapy in non-seasonal depressive episodes. The treatment of comorbid depressive disorder and borderline personality disorder (BPD) is usually reported to be less successful than the treatment of patients without personality disorder. Studies describing the use of bright light in depressed patients with comorbid BPD have not been published so far.