Previous studies suggested that antidepressants augmented with second-generation antipsychotics (SGAs), including aripiprazole, olanzapine, quetiapine, and risperidone, resulted in better treatment response or higher rates of remission in patients with major depressive disorder (MDD). However, population-based study on SGA augmentation for patients with MDD remains limited. The purpose of this study was to investigate the effectiveness of SGA augmentation for treatment of MDD using the National Health Insurance Research Database in Taiwan.