The purpose of this study is to determine whether quetiapine when used as adjunct to lithium or divalproex is safe and effective in the maintenance treatment of adult patients with Bipolar I Disorder. The study consists of enrollment and 2 phases, the Open-label treatment Phase and the Randomized treatment Phase.

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 diagnosis of Bipolar I Disorder, Most recent episode Manic (296.4x), or Bipolar I Disorder, Most Recent Episode Depressed (296.5x), or Bipolar I Disorder, Most recent Episode Mixed (296.6x), with or without psychotic features, as defined by Diagnostic and Statistical Manual of Mental Disorders - Fourth Edition (DSM-IV)
• At least 1 manic, depressed, or mixed episode in the 2 years prior to the index episode.
• Able to understand and comply with the requirements of the study.

Exclusion Criteria:

• Diagnosis of an anxiety disorder as defined by DSM-IV, which was treated with medication within the past year.
• Known intolerance or lack of response to quetiapine fumarate or to the assigned mood stabilizer, as judged by the investigator.
• Previously randomized into this study or D1447C00126