Standard pharmacotherapy for the maintenance treatment of patients with bipolar I disorder consists of lithium, valproate, or carbamazepine. However, many patients fail to respond to monotherapy with any of these agents, and as a result, psychiatrists often resort to polypharmacy. Findings from some open-label trials and retrospective chart reviews suggest this approach may be useful, but in the few controlled trials that have been conducted, the results have been negative. One drug combination that warrants further study as maintenance therapy is lithium plus valproate. Each is approved by the U.S. Food and Drug Administration for treatment of acute mania, and lithium has demonstrated efficacy for maintenance treatment as well. Some preliminary evidence suggests that the combination can be effective for patients who do not respond to monotherapy, and it seems to be no more dangerous than monotherapy. Concomitant administration of lithium plus valproate does not significantly alter lithium pharmacokinetics, and statistically significant changes that arise in valproate pharmacokinetics are not clinically significant. Although it is not known whether the drugs interact to augment response, many of their effects in the central nervous system do differ, and there is no indication of pharmacodynamic interactions that oppose each other. Finally, some evidence suggests that lithium and valproate may differ with regard to clinical variables that predict response to treatment.