OBJECTIVES: [corrected] To assess efficacy and safety of gabapentin in the treatment of bipolar disorder. METHODS: This was a double-blind, placebo-controlled trial of adjunctive gabapentin (dosed flexibly between 900 and 3,600 mg/day). Patients with a lifetime diagnosis of bipolar disorder (type I), and who were currently suffering from symptoms of either mania, hypomania or a mixed state despite ongoing therapy with lithium, valproate, or lithium and valproate in combination were eligible for inclusion. The primary efficacy measures were the baseline to endpoint change in total score on the Young Mania Rating Scale (YMRS) and the Hamilton Depression Rating Scale (HAM-D). RESULTS: Both treatment groups had a decrease in total YMRS from baseline to endpoint, but this decrease was significantly greater in the placebo group (-9) than the gabapentin group (-6) (p < 0.05). No difference between treatments was found for the total score on the HAM-D. Secondary efficacy measures were not different between treatment groups. More patients in the placebo group had changes made to their ongoing lithium therapy (n = 12) compared to the gabapentin group (n = 4). When these patients are removed from the efficacy analysis, the YMRS treatment difference still favors placebo, but is no longer statistically significant. Based on gabapentin plasma levels at termination, some patients did not take the study drug as prescribed. CONCLUSIONS: The findings of this study did not demonstrate that gabapentin is an effective adjunctive treatment when administered to outpatients with bipolar disorder.