There are a number of effective interventions for the treatment of depression. It is possible that the efficacy of these treatments will be improved further by the use of adjunctive therapies such as folate.1. To determine the effectiveness of folate in the treatment of depression 2. To determine the adverse effects and acceptability of treatment with folate.The Cochrane Controlled Trials Register (CCTR), and the Cochrane Collaboration Depression, Anxiety and Neurosis Controlled Trials Register (CCDANCTR) incorporating results of group searches of EMBASE, MEDLINE, LILACS, CINAHL, PSYNDEX and PsycLIT were searched. Reference lists of relevant papers and major textbooks of affective disorder were checked. Experts in the field and pharmaceutical companies were contacted regarding unpublished material.All randomised controlled trials that compared treatment with folic acid or 5'-methyltetrahydrofolic acid to an alternative treatment, whether another antidepressant medication or placebo, for patients with a diagnosis of depressive disorder (diagnosed according to explicit criteria).Data were independently extracted from the original reports by two reviewers. Statistical analysis was conducted using Review Manager version 4.1.Three trials involving 247 people were included. Two studies involving 151 people assessed the use of folate in addition to other treatment, and found that adding folate reduced Hamilton Depression Rating Scale scores on average by a further 2.65 points (95% confidence interval 0.38 to 4.93). Fewer patients treated with folate experienced a reduction in their HDRS score of less than 50% at ten weeks (relative risk (RR) 0.47, 95% CI 0.24 to 0.92) The number needed to treat with folate for one additional person to experience a 50% reduction on this scale was 5 (95% confidence interval 4 to 33). One study involving 96 people assessed the use of folate instead of the antidepressant trazodone and did not find a significant benefit from the use of folate. The trials identified did not find evidence of any problems with the acceptability or safety of folate.The limited available evidence suggests folate may have a potential role as a supplement to other treatment for depression. It is currently unclear if this is the case both for people with normal folate levels, and for those with folate deficiency.