The purpose of this study is to determine that the efficacy of the sustained release (SR) formulation of quetiapine (Seroquel) is not inferior to the immediate release (IR) formulation.

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:

• A stable dose of quetiapine IR between 300 and 800 mg/day within 4 weeks prior to the enrolment visit (Visit 1) as judged by the investigator.
• Female patients of childbearing potential must have a negative serum pregnancy test at enrolment and be willing to use a reliable method of birth control, ie, barrier method, oral contraceptive, implant, dermal contraception, long-term injectable contraceptive, intrauterine device, or tubal ligation during the study.
• Able to understand and comply with the requirements of the study, as judged by the investigator.

Exclusion Criteria:

• Meeting the criteria for any other (than schizophrenia) Diagnostic and Statistical Manual of Mental Disorders, 4th Edition (DSM-IV) Axis I diagnosis, concomitant organic mental disorder or mental retardation.
• Patients with substance abuse or dependence, as defined by DSM-IV, and not in full remission. A urine drug screen test will be performed. The investigator will evaluate the results along with medical history to determine if the patient meets DSM-IV criteria for substance abuse or dependence.
• Risk of transmitting human immunodeficiency virus (HIV) or hepatitis B, via blood or other body fluids, as judged by the investigator.
• Patients who, in the opinion of the investigator, pose an imminent risk of suicide or a danger to self or others.