Patients with chronic or recurrent major depressive disorder (MDD) have faced a dearth of treatment options. The present study evaluated the effectiveness and tolerability of aripiprazole augmentation for the treatment of chronic or recurrent MDD. This was the first 12-week prospective, multicentre, open-label study of the effectiveness and tolerability of flexibly dosed aripiprazole as an augmentation to ongoing antidepressant treatment in patients with chronic or recurrent MDD. The primary outcome measure for effectiveness was changes between baseline and endpoint (week 12) in total scores on the Montgomery–Asberg Depression Rating Scale. Adverse events (AEs) occurring throughout the trial are also reported. The Montgomery–Asberg Depression Rating Scale total scores decreased significantly between the baseline and the endpoint (magnitude of difference=−11.6, P<0.0001). At the endpoint, the response rate was 55.2% and the remission rate was 41.3%. Adjunctive aripiprazole treatment administered from week 1 through the endpoint was associated with remission and significant treatment responses. More than half (55.8%) of those taking adjunctive aripiprazole completed the study and relatively few patients discontinued participation because of AEs. None of the patients discontinued participation in the study because of an inadequate therapeutic response. Common AEs included headache, akathisia, insomnia and constipation. The mean dose of aripiprazole at the endpoint was 6.6 mg/day. Adjunctive aripiprazole may be effective and tolerable for patients with chronic or recurrent MDD. Adequately powered and controlled clinical trials should be conducted to confirm our open-label study findings.