An emerging body of evidence supports a role for dysfunctional purinergic neurotransmission in mood disorders. Adenosine agonists have been shown to have properties similar to those of dopamine antagonists; there is a well-characterized interaction between adenosine and dopamine receptors in the ventral striatum, and increasing adenosinergic transmission has been demonstrated to reduce the affinity of dopamine agonists for dopamine receptors. Allopurinol increases adenosine levels in the brain, and hence is hypothesized to reduce the symptoms of mania. Two randomized, placebo-controlled trials administering add-on allopurinol to manic patients showed significantly greater improvements in Young Mania Rating Scale (YMRS) scores for drug compared to placebo, while a more recent, relatively small, add-on study showed negative results. Based on these data, our objective was to examine the efficacy of allopurinol as add-on treatment to mood stabilizers and/or antipsychotic agents in manic patients with bipolar disorder.