Findings of substantial remaining morbidity in treated major depressive disorder (MDD) led us to review controlled trials of treatments aimed at preventing early relapses or later recurrences in adults diagnosed with MDD to summarize available data and to guide further research. Reports (n=97) were identified through systematic, computerized literature searching through February, 2015. Treatment versus control outcomes were summarized by random-effects meta-analysis. In 45 reports of 72 trials (N=14,450 subjects) lasting 33.4 weeks, antidepressants were more effective than placebo in preventing relapses (RR=1.90 [CI: 1.73–2.08]; NNT=4.4; p