Objective To assess the relationship between IFN-γ and psoriasis. Methods We searched The Cochrane Library (Issue 3, 2005), MEDLINE (1996 to 2005) and the China Biological Medicine Database (1978 to 2005). The search was conducted in November 2005. The quality of included clinical controlled trials, case studies and cohort studies was evaluated independently by three reviewers. RevMan 4.2.8 software was used. Results In total, 23 studies were included, involving 612 psoriasis patients and 441 healthy controls. All studies did not provide sufficient detail, on the random sampling and the specificity of the kits used for the analyses. Compared with the controls, the serum or plasma IFN-γ in psoriasis patients showed significantly higher levels (SMD=0.89, 95%CI 0.29 to 1.48; and RR=6.20, 95%CI 1.78 to 21.61). The concentration of IFN-γ in supernatant obtained from cultured cells showed slightly higher levels (SMD=0.99, 95%CI -0.01 to 1.99; and RR=5.54, 95%CI 2.03 to 15.13). Conclusion The evidence currently available shows that the increase of IFN-γ may be relevant to psoriasis. However, these results could be affected by the high risk of selection, confounding and detection bias of included studies. More persuasive evidence, from high quality studies, is needed.