This study is designed to determine the safety and effectiveness of vilazodone for major depressive disorder and to discover genetic markers associated with response. This study will enroll approximately 400 patients.

This randomized, double-blind, placebo-controlled, multicenter, 8-week, clinical trial is designed to assess the efficacy and safety of vilazodone and to discover genetic markers of treatment response associated with vilazodone use in adult patients diagnosed with MDD by the DSM-IV-TR criteria. This study will enroll approximately 400 patients.

Inclusion Criteria:

• Male or female patients 18-65 years of age, inclusive.
• A diagnosis of MDD, single episode or recurrent, according to DSM-IV-TR (296.2/296.3) with a current Major Depressive Episode of less than two year's duration with a minimum duration of at least 4 weeks.
• HAM-D score ≥ 22.
• HAM-D item 1 (depressed mood) score ≥ 2.
• Patients must be able to provide written informed consent to participate before beginning any trial related activities.
• Patients must be able to speak, read and understand English and possess the ability to respond to questions and follow simple instructions.

Exclusion Criteria:

• A current (or within 6 months prior to the Screening Visit) Axis I disorder of Post Traumatic Stress Disorder, Eating Disorder, Obsessive Compulsive Disorder (Generalized Anxiety Disorder, Social Phobia or Simple Phobia will be allowed).
• A history of schizophrenia, schizoaffective disorder or bipolar I or II disorder (with a history of hypomanic or manic episodes).
• DSM-IV-TR criteria for substance abuse (alcohol or drugs) within 3 months prior to Screening Visit or substance dependence within 6 months prior to the Screening Visit.
• Criteria for any of the following DSM-IV-TR MDD Specifiers: [a] With Catatonic Features; [b] With Postpartum Onset; [c] With Seasonal Pattern.