The purpose of this study is to evaluate the efficacy of quetiapine SR in combination with an antidepressant versus an antidepressant alone in patients with MDD with inadequate response to an antidepressant treatment.

PLEASE NOTE: Seroquel SR and Seroquel XR refer to the same formulation. The SR designation was changed to XR after consultation with FDA.

Inclusion Criteria:

• Patient is able to provide written informed consent before beginning any study related procedures
• Patient has a documented clinical diagnosis of major depressive disorder
• Patient is able to understand and comply with the requirements of the study, as judged by a study investigator

Exclusion Criteria:

• Patients with a history of non-compliance as judged by the study investigator
• Patients with a known lack of response to previous treatment with quetiapine
• Patients who have participated in a clinical trial within 4 weeks of randomization