Participants in this study are treated with lithium, divalproex (Depakote), and one of the following atypical antipsychotic medications: olanzapine (Zyprexa), risperidone (Risperdal) or quetiapine (Seroquel) for at least 24 weeks. Participants who have already begun combination therapy with at least one of the mood stabilizers and atypical antipsychotic medications listed above are also encouraged to enroll in this study. After participants have been on combination therapy for at least 24 weeks they will then be randomly assigned to one of two groups. The first group will continue to receive active mood stabilizer and atypical antipsychotic medication. The second group will receive active mood stabilizer and placebo. Participants are assessed weekly and followed for up to 18 months.