Objective To study the role of IL-17 in the pathogenesy and differential diagnosis of the 3 types of CNS infections (tubercular meningitis [TBM], cryptococcal meningitis [CM], viral meningoencephalitis [VM]). Methods One hundred and twelve patients with CNS infections,admitted to out hospital from February 2008 to December 2009, were chosen and divided into TBM group, CM group and VM group;another 36 patients without CNS infections either autoimmune disease at the same period were chosen as control group. ELISA was employed to determine the levels of IL-17,IL-12, IFN-γ in the cerebrospinal fluid in these 4 groups. Results IL-17 level in the cerebrospinal fluid of the 3 groups with CNS infections were obviously different, but all of them were significantly higher than that in control group (P＜0.05), with CM group enjoying the highest level, following by TBM group,VM group and control group. The levels of IL-12 and IFN-γ in patients with TBM and CM were higher than those in patients with VM and controls. Through the correlation analysis of IL-17, IL-12 and IFN-γ in patients with TBM, we found that IL-17 level was negatively correlated with IL-12 level (r=-0.3 11,P=0.033);the levels of these 3 cytokines were positively correlated with the quantity of leucocytes (r=0.219, 0.434 and 0.341, P=0.031, 0.027 and 0.001). ROC curves were established for differential diagnosis among these 3 groups according to CSF IL-17 levels, and all of the areas under the curve were bigger than 0.7. Conclusion Thl7 pathway is widely involved in the immune responses of CNS infection, and interacts with Thl pathway. The different levels of IL-17 in the cerebrospinal fluids in patients with various CNS infections may contribute to differential diagnosis.