The efficacy and tolerability of Lu AA21004 in the prevention of relapse of major depressive disorder (MDD) in patients in remission after acute treatment was evaluated. Patients (n=639) aged 18-75 years with a primary diagnosis of MDD with a current major depressive episode (MDE) ≥4 weeks' duration, at least one prior MDE and a MADRS total score ≥26 received 12-week, open-label Lu AA21004 at 5 or 10mg/day. Patients in remission (MADRS ≤10) at both weeks 10 and 12 were assigned to double-blind treatment with either placebo or Lu AA21004 (fixed dose from Week 8).Patients (n=396) were treated, after random assignment to placebo (n=192) or Lu AA21004 (n=204). The primary analysis of time to relapse (full-analysis set, Cox proportional hazard model) showed a statistically significant difference in favour of Lu AA21004 versus placebo with a hazard ratio of 2.01 (95% confidence interval: 1.26-3.21; p=0.0035). The proportion of patients who relapsed was 13% in the Lu AA21004 group (n=27) and 26% in the placebo group (n=50). The withdrawal rates due to adverse events were 8% (open-label), and 3% (placebo) and 8% (Lu AA21004) (double-blind). Thus, Lu AA21004 was effective in preventing relapse of MDD and was well tolerated as maintenance treatment.