 The clinical use of clozapine has been an unequivocal advance in the treatment of schizophrenia, a chronic and severe mental illness. A wealth of clinical data demonstrates it offers enhanced efficacy on both positive and negative symptomatology, improving cognition, functioning and quality of life. It is also associated with improved compliance and a continued efficacy in long-term treatment that can be translated into a reduction of suicidality and all-cause mortality. Because of preclinical evidence that it modulates neuroplasticity and prefrontal cortex connectivity, clozapine may be an interesting strategy for further severe psychotic illnesses. Nevertheless, even considering the growing use of other atypical antipsychotics in the management of bipolar disorder, a role for clozapine has been poorly defined. The clinical evidence-base for its use in this condition is largely based on uncontrolled naturalistic trials and retrospective studies and chart reviews. Several of these have supported clozapines efficacy in treatment-resistant bipolar disorder. Possibly because of clozapines profile of adverse effects and lack of interest from pharmaceutical companies, only two randomized trials have examined its effectiveness. Both suggest clinically relevant antimanic and mood-stabilizing properties. Therefore, the primary objective of this trial is to determine the effectiveness of clozapine for treatment-resistant bipolar disorder. Secondary objectives include examining the effects of treatment with clozapine on cognition and functioning of patients with bipolar disorder. Tolerability and safety of long-term clozapine use will also be examined. To that end, the investigators will conduct a clinical trial with 54 patients with a history of treatment resistance. Patients will be randomized to either open-label treatment with clozapine, in combination with lithium or valproate, or open-label treatment with an atypical antipsychotic with consistent evidence of efficacy in the treatment of bipolar disorder olanzapine, quetiapine or risperidone, also in combination with lithium or valproate. Patients will be followed for one-year and time to all-cause treatment failure will be the primary outcome measure. It is the belief of the investigators that this study will generate meaningful clinical data of tremendous importance to validate clozapine as a legitimate treatment option for treatment-resistant bipolar disorder. 