mda-7/IL-24 has tumor-suppressor activity in a broad spectrum of human cancer cells. However, the therapeutic effect of the recombinant human IL-24 protein on human gallbladder carcinoma has rarely been explored. In this study, we used a human gallbladder carcinoma cell line (GBC-SD) to explore the effect of adenovirus-mediated IL-24 (Ad-IL24) gene therapy on GBC-SD cells. We show that Ad-IL24 treatment of GBC-SD cells in vitro conspicuously induced apoptosis of GBC-SD cells. We also demonstrate that the in vivo treatment of GBC tumor-bearing athymic nude mice intratumorally injected with Ad-IL24 significantly suppressed GBC growth. To further explore the mechanism that mda-7/IL-24 utilized in tumor cell apoptosis, we examined molecules and pathways involved in apoptotic regulation and found that Ad-IL24 induced the down-regulation of anti-apoptotic gene Bcl-2 and the release of cytochrome c, which subsequently activated caspase-9, caspase-3 and PARP to induce apoptosis. In summary, adenovirus (AdV)-mediated IL-24 overexpression exerted potent antitumor activity via stimulating mitochondrial apoptotic pathway in GBC-SD. Therefore, mda-7/IL-24 has the potential to serve as a tool for targeted gene therapy in the treatment of gallbladder cancer.