Inclusion Criteria:

• Male or female subjects, 4-6 years of age.
• Subjects must have a DSM-IV diagnosis of bipolar I, bipolar II disorder or bipolar spectrum disorder and currently displaying manic, hypomanic, or mixed symptoms (with or without psychotic features) according to the DSM-IV based on clinical assessment and confirmed by structured diagnostic interview (Kidd Schedule of Affective Disorders and Schizophrenia Epidemiological Version). Bipolar spectrum disorder (or sub-threshold bipolar disorder) is operationalized as having severe mood disturbance, which meets DSM-IV Criteria A for bipolar disorder but meet fewer elements in criteria B (only require 2 items for elation category and 3 for irritability).
• Subjects and their legal representative must have a level of understanding sufficient to communicate intelligently with the investigator and study coordinator, and to cooperate with all tests and examinations required by the protocol.
• Subjects and their legal representative must be considered reliable.
• Each subject and his/her authorized legal representative must understand the nature of the study. The subject's authorized legal representative must sign an informed consent document.
• Subjects must have an initial score on the Y-MRS total score of at least 20.
• Subject must be able to participate in mandatory blood draws.
• Subjects with comorbid ADHD, ODD, CD, anxiety and depressive disorders will be allowed to participate in the study provided they do not meet for any of the exclusionary criteria.
• For concomitant stimulant therapy used to treat ADHD, subjects must have been on a stable dose of the medication for 1 month prior to study enrollment. The dose of the stimulant therapy will not change throughout the duration of the study.

Exclusion Criteria:

• Investigator and his/her immediate family; defined as the investigator's spouse, parent, child, grandparent, or grandchild.
• Serious, unstable illness including hepatic, renal, gastroenterological, respiratory, cardiovascular (including ischemic heart disease), endocrinologic, neurologic, immunologic, or hematologic disease.
• Uncorrected hypothyroidism or hyperthyroidism.
• History of severe allergies or multiple adverse drug reactions.
• Non-febrile seizures without a clear and resolved etiology.
• Leukopenia or history of leukopenia without a clear and resolved etiology.
• Judged clinically to be at serious suicidal risk.
• Any other concomitant medication with primarily central nervous system activity other than specified in Concomitant Medication portion of the protocol.
• A non-responder or a history of intolerance to an adequate trial of Quetiapine(2 months or more at an adequate dose) as determined by the clinician.
• Current diagnosis of schizophrenia.
• Non English speaking subjects will not be allowed into the study for the following reasons: a) the assessment instruments are not available and have not been adequately standardized in other languages; b) our clinical trials facility is located in Cambridge and not in the MGH main campus without the availability of translators; c) psychiatric questionnaires and evaluations are taxing and adding the complexity of a translator has the potential to make the patient experience even more exhausting.