To investigate the role of IL-18 and IL-18BP balance in the spleens of patients with primary immune thrombocytopenia (ITP).A total of 12 active ITP patients and 10 normal controls were recruited. Their expressions of IL-18 and IL-18BP were measured by immunohistochemistry (IHC) and immunofluorescence. The mRNA expressions of IL-18, IL-18BP, IFN-γ and IL-4 were studied by reverse transcription-polymerase chain reaction (RT-PCR) in all subjects.The positive rate of IL-18 was higher in the ITP patients than that of the normal control group (100% vs 90.0%). And the positive rate of IL-18BP was higher in the ITP patients than that of the normal control spleen (83.3% vs 60.0%). The relative amount of mRNA gene expression of IL-18 was increased 6.0-fold in active patients compared to controls (P<0.05). But the elevated mRNA content of IL-18BP failed to compensate for the sharply elevated content of IL-18. As compared with the normal control group, the expression level of IFN-γ mRNA expression was significantly increased 10.2-fold in ITP patients (P<0.05). The decrease observed in IL-4 was 41.7% in active patients compared to controls (P<0.05).The IL-18/IL-18BP imbalance plays an important role in pathogenesis of ITP and its correction may become a potential therapeutic target for ITP.