It has been demonstrated that interleukin 18 (IL-18) exerts antitumor activity. In this study, we investigated whether oncolytic adenovirus-mediated gene transfer of IL-18 could induce strong antitumor activity. A tumor-selective replicating adenovirus expressing IL-18 (ZD55-IL-18) was constructed by insertion of an IL-18 expression cassette into the ZD55 vector, which is based on deletion of the adenoviral E1B 55-kDa gene. ZD55-IL-18 could express substantially more IL-18 than Ad-IL-18 because of replication of the vector. It has been shown that ZD55-IL-18 exerted a strong cytopathic effect and significant apoptosis in renal cell carcinoma. ZD55-IL-18 significantly decreased VEGF and CD34 expression in the tumor cells. Treatment of established tumors with ZD55-IL-18 showed much stronger antitumor activity than that induced by ZD55-EGFP or Ad-IL-18. These data indicated that oncolytic adenovirus expressing IL-18 could exert potential antitumor activity via inhibition of angiogenesis and offer a novel approach to cancer therapy.