The purpose of this study is to investigate the efficacy of allopurinol as an augmentation agent for the prevention of mania in bipolar disorder patients with currently stable mood. Bipolar disorder is a mental disorder with severe mood swings (mania and depression). Despite the development of medications for mania, they may have significant side effects, high costs and the need for serum level monitoring. These factors adversely affect medication compliance in bipolar patients. One recent study indicated the efficacy of allopurinol in the treatment of bipolar mania. Our hypothesis is that the addition of allopurinol to standard medications for bipolar disorder will decrease the recurrences of manic episodes compared to standard medications.