The functional polymorphism of the brain-derived neurotrophic factor (BDNF) in relation to suicidal behaviour has attracted a great deal of interest in recent research. Although genetic studies have indicated BDNF as a candidate gene in suicidal behaviour, no clear evidence exists for the role of BDNF levels in the blood of patients with mental illness who are at risk of suicide. Considering the ability of BDNF to cross the blood-brain barrier, the aim of the present study was to review evidence for a correlation between blood BDNF levels and suicidal behaviour among patients with psychiatric disorders. The systematic review that was performed (1966 - 2012) identified 64 studies as potential candidates for inclusion. After scrutiny, only seven studies appeared to focus on BDNF levels in plasma, serum and platelets. Studies consistently showed a significant decrease in BDNF levels among patients with previous suicide attempts, with the exception of one study, which included patients with schizophrenia. No significant differences were found between BDNF levels, gender and lethality of suicide attempts. Further evidence is required for which blood sample type to use when examining BDNF protein levels in terms of suicidal behaviour in mental illness sufferers, and more focus should be given to a potential blood BDNF threshold among patients at suicide risk.