This randomized withdrawal study assessed relapse prevention with vilazodone in adults with major depressive disorder. After 20 weeks of open-label treatment with vilazodone 40 mg/day, responders were randomized (1 : 1 : 1) to 28 weeks of double-blind, fixed-dose treatment with vilazodone 20 mg/day, vilazodone 40 mg/day, or placebo. The primary efficacy endpoint was time to first relapse, defined as Montgomery–Åsberg Depression Rating Scale total score of at least 18 and meeting major depressive episode criteria, Montgomery–Åsberg Depression Rating Scale total score of at least 18 at two consecutive visits, or discontinuation for an insufficient therapeutic response. Of 1204 patients who received open-label treatment, 564 completed treatment and were randomized (placebo=192, vilazodone 20 mg/day=185, vilazodone 40 mg/day=187). No significant difference was detected in time to relapse during the double-blind period (P>0.05). The crude percentage of patients that relapsed was similar between treatment groups (placebo=12.6%; vilazodone 20 mg/day=11.4%; vilazodone 40 mg/day=13.4%). The most common treatment-emergent adverse events were diarrhea (29.6%), nausea (24.0%), and headache (14.0%) during open-label treatment and headache (8.9%), nasopharyngitis (8.4%), and diarrhea (7.5%) during double-blind treatment in the combined vilazodone groups (20 and 40 mg/day). In conclusion, time to relapse with vilazodone was not statistically different from placebo. Vilazodone was generally well tolerated in adults with major depressive disorder.