BACKGROUND: Seasonal affective disorder (SAD) is a seasonal pattern of recurrent major depressive episodes that most commonly occurs during autumn or winter and remits in spring. The prevalence of SAD in the United States ranges from 1.5% to 9%, depending on latitude. The predictable seasonal aspect of SAD provides a promising opportunity for prevention. This is one of four reviews on the efficacy and safety of interventions to prevent SAD; we focus on agomelatine and melatonin as preventive interventions. OBJECTIVES: To assess the efficacy and safety of agomelatine and melatonin (in comparison with each other, placebo, second-generation antidepressants, light therapy, psychological therapy or lifestyle interventions) in preventing SAD and improving patient-centred outcomes among adults with a history of SAD. SEARCH METHODS: We conducted a search of the Specialised Register of the Cochrane Depression, Anxiety and Neurosis Review Group (CCDANCTR) to 11 August 2015. The CCDANCTR contains reports of relevant randomised controlled trials from EMBASE (1974 to date), MEDLINE (1950 to date), PsycINFO (1967 to date) and the Cochrane Central Register of Controlled Trials (CENTRAL). Furthermore, we searched the Cumulative Index to Nursing and Allied Health Literature (CINAHL), Web of Knowledge, The Cochrane Library and the Allied and Complementary Medicine Database (AMED) (to 26 May 2014). We conducted a grey literature search (e.g. in clinical trial registries) and handsearched the reference lists of all included studies and pertinent review articles. SELECTION CRITERIA: To examine efficacy, we planned to include randomised controlled trials (RCTs) on adults with a history of winter-type SAD who were free of symptoms at the beginning of the study. To examine adverse events, we intended to include non-randomised studies. We planned to include studies that compared agomelatine versus melatonin, or agomelatine or melatonin versus placebo, any second-generation antidepressant (SGA), light therapy, psychological therapies or lifestyle changes. We also intended to compare melatonin or agomelatine in combination with any of the comparator interventions listed above versus the same comparator intervention as monotherapy. DATA COLLECTION AND ANALYSIS: Two review authors screened abstracts and full-text publications against the inclusion criteria. Two review authors planned to independently extract data and assess risk of bias of included studies. We planned to pool data for meta-analysis when participant groups were similar and when studies assessed the same treatments by using the same comparator and presented similar definitions of outcome measures over a similar duration of treatment; however, we identified no studies for inclusion. MAIN RESULTS: We identified 2986 citations through electronic searches and reviews of reference lists after de-duplication of search results. We excluded 2895 records during title and abstract review and assessed 91 articles at full-text level for eligibility. We identified no controlled studies on use of melatonin and agomelatine to prevent SAD and to improve patient-centred outcomes among adults with a history of SAD. AUTHORS' CONCLUSIONS: No available methodologically sound evidence indicates that melatonin or agomelatine is or is not an effective intervention for prevention of SAD and improvement of patient-centred outcomes among adults with a history of SAD. Lack of evidence clearly shows the need for well-conducted, controlled studies on this topic. A well-conducted RCT of melatonin or agomelatine for prevention of SAD would assess the comparative benefits and risks of these interventions against others currently used to treat the disorder.