This is a multicenter, randomized, parallel-group, double-blind, placebo-controlled study to evaluate the efficacy and safety of quetiapine and lithium (comparator in the study) for up to 104 weeks of maintenance treatment in adult patients with Bipolar I Disorder.

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:

• Signed informed consent
• 18 years or older
• A diagnosis of Bipolar I Disorder
• Have a current manic, depressed or mixed episode
• Have had a manic, depressed or mixed episode during the last 26 weeks that was treated with quetiapine
• Female patients of childbearing potential must be using a reliable method of contraception

Exclusion Criteria:

• Pregnancy
• Substance or alcohol dependence at enrollment
• Unstable thyroid function
• Unstable Diabetes
• Unstable or inadequately treated medical illness e.g., angina pectoris and hypertension
• Use of an experimental drug within 30 days of enrollment