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Objective: Lithium and valproate are commonly used either in monotherapy or in combination with atypical antipsychotics in maintenance treatment of bipolar I disorder; however, their comparative efficacy is not well understood. This study aimed to compare the efficacy of valproate and lithium on mood stability either in monotherapy or in combination with atypical antipsychotics., Methods: We performed a post hoc analysis using data from a 52-week randomized double-blind, placebo-controlled trial, that recruited 159 patients with recently remitted mania during treatment with lithium or valproate and adjunctive atypical antipsychotic therapy. Patients were randomized to discontinue adjunctive atypical antipsychotic at 0, 24 or 52 weeks., Results: No significant differences in efficacy were observed between valproate and lithium (hazard ratio: 0.99; 95% confidence interval: [0.66, 1.48]) in time to any mood event. Valproate with 24 weeks of atypical antipsychotic was significantly superior to valproate monotherapy in preventing any mood relapse (hazard ratio: 0.46; 95% confidence interval: [0.22, 0.97]) while lithium with 24 weeks of atypical antipsychotic was superior to lithium monotherapy in preventing mania (hazard ratio: 0.27; 95% confidence interval: [0.09, 0.85]) but not depression., Conclusion: Overall, this study did not find significant differences in efficacy between the two mood-stabilizing agents when used as monotherapy or in combination with atypical antipsychotics. However, study design and small sample size might have precluded from detecting an effect if true difference in efficacy existed. Further head-to-head investigations with stratified designs are needed to evaluate maintenance therapies.