The purpose of this study is to demonstrate superior efficacy of Quetiapine fumarate sustained release (SEROQUEL) compared with placebo when used as a mono-therapy in the treatment of Major Depressive Disorders 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