To a large extent, variability in antidepressant efficacy can be explained by genetic variations that affect medication-metabolizing enzymes, drug transporters, and medication targets. Recent reviews demonstrated significant potential of pharmacogenomic testing in improving treatment of major depressive disorder. One of the major barriers towards successful implementation of pharmacogenomic testing for patients with major depressive disorder is lack of systematic evaluation of impact of this approach in routine clinical care settings. The major goal of this study is to systematically evaluate impact of comprehensive pharmacogenomic testing on the treatment of major depressive disorder in ambulatory setting.