Objectives: This review assesses the evidence about the effects of detention on the mental and physical health and social functioning of asylum seekers. Method and Analysis: We followed Campbell Collaboration guidelines to conduct a systematic review. Meta-analytic methods were used to quantitatively synthesize the study results. Results: Primary study effect sizes for post-traumatic stress disorder, depression, and anxiety, while the asylum seekers were still detained lies in the range 0.35-0.99, all favoring the nondetained asylum group. Author's Conclusions: There is some evidence to suggest an independent adverse effect of detention on the mental health of asylum seekers. The conclusions should however be interpreted with caution as they are based on few studies. More research is needed in order to fully investigate the effect of detention on mental health.