The study is using a randomized double-blind, parallel-group, placebo-controlled design. 30 patients per treatment group will be included into the study and randomized to the treatment groups using a computer program. 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, 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 a double-blind therapy of either 10 mg/d olanzapine or placebo. Study visits will be performed on days 4, 7, and 14 (visits 2-4: vital signs, HAMD, MADRS, CGI, lab).