IL-18 is a pleiotropic cytokine that is produced by many cancer cells. A recent report suggested that IL-18 plays a key role in regulating the immune escape of melanoma and gastric cancer cells. Thrombospondin (TSP-1) is known to inhibit angiogenesis in several cancers but some studies have reported that it stimulates angiogenesis in some cancers such as gastric cancer. IL-18 and TSP-1 are related to tumor proliferation and metastasis. This study investigated the relationship between IL-18 and TSP-1 in gastric cancer. RT-PCR and ELISA showed that after the cells had been treated with IL-18, the level of TSP-1 mRNA expression and TSP-1 protein production by IL-18 increased in both a dose- and time-dependent manner. The cells were next treated with specific inhibitors in order to determine the signal pathway involved in IL-18-enhanced TSP-1 production. IL-18-enhanced TSP-1 expression was blocked by SP600125, a c-Jun N-terminal kinase (JNK) specific inhibitor. In addition, Western blot showed that IL-18 enhanced the expression of phosphorylated JNK. Overall, these results suggest that IL-18 plays a key role in TSP-1 expression involving JNK.