AIM: To investigate the expression of macrophage migration inhibitory factor (MIF) and detect its role in the innate immune response of fungal keratitis (FK). METHODS: We collected the paraffin-embedded cornea tissues from 10 FK and 6 ocular trauma patients to explore the MIF expression by immunohistochemistry. Then we cultured telomease-immortalized human corneal epithelial cells (THCEs), stimulated by the hyphae suspension of Aspergillus fumigatus (A. fumigatus) to detect the change of MIF with or without the pretreatment of MIF inhibitor [4-Iodo-6-phenylpyrimidine (4-IPP)] by real-time polymerase chain reaction (PCR). The protein level of MIF was also tested by immunohistochemistry, and the level of tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) mRNA were compared between normal, hyphae stimulated and 4-IPP pretreated groups by real-time PCR to study the influence of MIF on the expression of TNF-α and IL-6. Corneal severity of rats’ FK models was documented by clinical scores, and real-time PCR. Western blot and immunohistochemistry were used to test the expression of MIF, TNF-α and IL-6 in rats’ corneas. RESULTS: In the corneas of FK patients, there was much stronger expression of MIF than that in the normal group showed by immunohistochemistry. In cultured THCEs stimulated by A. fumigatus, the expression of MIF became stronger in both immunohistochemistry and PCR at 16, 24, 32 and 48h post infection (p.i.; P<0.01, P<0.01, P<0.01, P<0.05). After pretreated with 4-IPP, the expression of MIF reduced at 4, 8, 16h p.i. (P<0.05, P<0.05, P<0.05) and the downstream TNF-α and IL-6 decreased obviously (P<0.05, P<0.01). In rats with A. fumigatus keratitis, the relative mRNA and protein level of MIF increased than those in the normal group by PCR (at 1d: P<0.01, 3d: P<0.01, 5d: P<0.01), Western blot and immunohistochemistry. After blocked MIF with 4-IPP, the clinical outcomes of rat keratitis showed markedly reduced inflammatory response (P<0.01), with TNF-α and IL-6 decreased in accordance with those in THCEs by PCR (P<0.05, P<0.01). CONCLUSION: The expression of MIF increased significantly in FK patients, THCEs and rats stimulated by A. fumigatus. After blocked with 4-IPP, the expression of MIF reduced, and so did its downstream cytokines: TNF-α and IL-6. The inflammation reaction of the rats’ corneas lightened after pretreated with 4-IPP. MIF may play a role in the innate immune response of the corneal resistance against A. fumigatus.