Although most treatment research on bipolar disorder has focused on mania, depressive episodes occur more frequently among patients with bipolar disorder. Here, we report the results of 2 identically designed, 8-week, multicenter, randomized, double-blind, placebo-controlled studies (CN138-096 and CN138-146) to evaluate the efficacy and safety of aripiprazole monotherapy in outpatients with bipolar I disorder experiencing a major depressive episode without psychotic features. Patients were randomized to placebo or aripiprazole (initiated at 10 mg/d, then flexibly dosed at 5-30 mg/d based on clinical effect and tolerability). The primary end point was mean change from baseline to Week 8 (last observation carried forward) in the Montgomery-Asberg Depression Rating Scale total score. In Studies 1 and 2, respectively, 186 and 187 patients were randomized to aripiprazole, and 188 and 188 to placebo. Although statistically significant differences were observed during Weeks 1 to 6, aripiprazole did not achieve statistical significance versus placebo at Week 8 in either study in the change in Montgomery-Asberg Depression Rating Scale total (primary end point). In addition, despite early statistical separation on the Clinical Global Impressions Bipolar Version Severity of Illness-Depression score (key secondary end point), aripiprazole was not superior to placebo at end point. Aripiprazole was associated with a higher incidence of akathisia, insomnia, nausea, fatigue, restlessness, and dry mouth versus placebo. More patients discontinued with aripiprazole versus placebo in Study 1 (46.8% vs 35.1%) and Study 2 (41.2% vs 29.8%). Aripiprazole monotherapy-as dosed in this study design-was not significantly more effective than placebo in the treatment of bipolar depression at end point (Week 8).