Inclusion Criteria:

1. Provision of written informed consent
2. A diagnosis of Unipolar Depression by Diagnostic and Statistical Manual of Mental Disorders- Fourth Edition (DSM-IV-TR)
3. Females and males aged 19-65 years
4. Female patients of childbearing potential must be using a reliable method of contraception and have a negative urine human chorionic gonadotropin (HCG) test at enrolment
5. Able to understand and comply with the requirements of the study
6. Minimum 21-item HAM-D GRID score of 15
7. Prior treatment with therapeutic doses of an SSRI-type antidepressant for at least 6 weeks
8. Unsatisfactory response to treatment, as determined by clinician, for at least 6 weeks.

Exclusion Criteria:

1. Pregnancy or lactation
2. Any DSM-IV Axis I disorder not defined in the inclusion criteria
3. English as a second language
4. Patients who, in the opinion of the investigator, pose an imminent risk of suicide or a danger to self or others
5. Known intolerance or lack of response to quetiapine fumarate, as judged by the investigator
6. Use of any of the following cytochrome P450 3A4 inhibitors in the 14 days preceding enrolment including but not limited to: ketoconazole, itraconazole, fluconazole, erythromycin, clarithromycin, troleandomycin, indinavir, nelfinavir, ritonavir, fluvoxamine and saquinavir
7. Use of any of the following cytochrome P450 inducers in the 14 days preceding enrollment including but not limited to: phenytoin, carbamazepine, barbiturates, rifampin, St. John's Wort, and glucocorticoids
8. Administration of a depot antipsychotic injection within one dosing interval (for the depot) before randomisation
9. Substance or alcohol dependence at enrolment (except dependence in full remission, and except for caffeine or nicotine dependence), as defined by DSM-IV criteria
10. Opiates, amphetamine, barbiturate, cocaine, cannabis, or hallucinogen abuse by DSM-IV criteria within 4 weeks prior to enrolment
11. Medical conditions that would affect absorption, distribution, metabolism, or excretion of study treatment
12. Unstable or inadequately treated medical illness (e.g. diabetes, angina pectoris, hypertension) as judged by the investigator
13. Involvement in the planning and conduct of the study
14. Previous enrolment or randomisation of treatment in the present study.
15. Participation in another drug trial within 4 weeks prior enrolment into this study or longer in accordance with local requirements
16. Previous head injury, associated with loss of consciousness.
17. Neurological disorder.
18. Significant physical health problem
19. An absolute neutrophil count (ANC) of 1.5 x 109 per liter;

20. A patient with Diabetes Mellitus (DM) fulfilling one of the following criteria:

    • Unstable DM defined as enrollment glycosylated hemoglobin (HbA1c) >8.5%.
    • Admitted to hospital for treatment of DM or DM related illness in past 12 weeks.
    • Not under physician care for DM
    • Physician responsible for patient's DM care has not indicated that patient's DM is controlled.
    • Physician responsible for patient's DM care has not approved patient's participation in the study. Has not been on the same dose of oral hypoglycaemic drug(s) and/or diet for the 4 weeks prior to randomization. For thiazolidinediones (glitazones) this period should not be less than 8 Weeks.
    • Taking insulin whose daily dose on one occasion in the past 4 weeks has been more than 10% above or below their mean dose in the preceding 4 weeks Note: If a diabetic patient meets one of these criteria, the patient is to be excluded even if the treating physician believes that the patient is stable and can participate in the study.