This will be an open pilot study using pre-post comparison. 20 treatment-refractory patients with major depression diagnosed by DSM-IV will be included. Psychotic features of depression will be excluded by a score of 2 or less in the PANSS subscales P1, P3 and P6. Treatment resistance as defined by history of non-response to two antidepressants from different classes at an acceptable dose and period is confirmed retrospectively. If possible, treatment compliance should be confirmed by plasma level examination. After informed consent, visit 1 is performed on day 0 (inclusion criteria, history, demographics, physical examination, vital signs, HAMD, MADRS, CGI, BDI, lab). Study medication is started on day 1, the antidepressive therapy is continued at stable dose until the end of the study. Patients will receive an adjunctive augmentation therapy of 10 mg/d aripiprazole. Study visits will be performed on days 3, 7, 14 and 21 (visits 2-5: vital signs, HAMD, MADRS, CGI, BDI, lab). In addition to the HAM-D and MADRS scores, patients will perform a self-rating (BDI). Aripiprazole plasma levels will be measured at the end of the treatment period to assess compliance and to detect a putative correlation between drug plasma levels and efficacy.