This is a single center, open prospective study to evaluate the effectiveness of quetiapine monotherapy in the maintenance treatment of adolescent patients with a bipolar spectrum disorder (bipolar I and bipolar II disorders, cyclothymia, and bipolar disorder not otherwise specified [bipolar NOS]) for a minimum of 48 weeks. Patients will be screened (enrolment phase) either when already stabilized or during an acute episode of mania/hypomania or depression.