Inclusion Criteria:

1. Provide written informed consent before initiation of any study-related procedures.
2. A lifetime diagnosis of bipolar spectrum disorder: bipolar I and bipolar II disorders, cyclothymia, bipolar NOS, as defined by the Diagnostic and Statistical Manual of Mental Disorders - Fourth Edition (DSM-IV).
3. Male or female, between the ages of 12 and 20 years at enrolment.
4. No preventive treatment at least one month prior to enrolment.
5. Female patients of childbearing potential and who are sexually active must be using a reliable method of contraception. Reliable methods of contraception include hormonal contraceptives (e.g., oral contraceptive or long-term injectable or implantable hormonal contraceptive), double-barrier methods (e.g., condom and diaphragm, condom and foam, condom and sponge), intrauterine devices, and tubal ligation.
6. Able to understand and comply with the requirements of the study.

Exclusion Criteria:

1. Known intolerance or lack of response to quetiapine fumarate as judged by the investigator.
2. Pregnancy or lactation. Female patients of childbearing potential must have a negative urine human chorionic gonadotropin (HCG) test at enrolment.
3. History of substance or alcohol dependence within three months of enrolment (except for caffeine or nicotine dependence), as defined by DSM-IV criteria.
4. Opiates, amphetamine, barbiturate, cocaine, cannabis, or hallucinogen abuse by DSM-IV criteria within 4 weeks prior to enrolment.
5. Use of any of the following cytochrome P450 3A4 inhibitors in the 14 days preceding enrolment including but not limited to: ketoconazole, itraconazole, fluconazole, erythromycin, clarithromycin, troleandomycin, indinavir, nelfinavir, ritonavir, and saquinavir. Should a new drug be started, investigators will consult the pharmacy as this list is not exhaustive.
6. Use of any of the following cytochrome P450 inducers in the 14 days preceding enrolment including but not limited to: phenytoin, carbamazepine, barbiturates, rifampin, St. John's Wort, and glucocorticoids.
7. Current use of fluvoxamine, nefazodone, or grapefruit juice.
8. Thyroid stimulating hormone (TSH) concentration outside of the normal range.
9. Unstable or inadequately treated medical illness (e.g., diabetes, angina pectoris, hypertension) as judged by the investigator.
10. History of hepatic disease, or elevated hepatic enzymes at entry testing.
11. Medical conditions that would affect absorption, distribution, metabolism, or excretion of study treatment.
12. Use of an experimental drug within 30 days of enrolment.
13. Previous trials of maintenance therapy.