The phosphatidylinositol 3-kinase (PI3K)/Akt cascade has an important role in the resistance of ovarian cancer cells to cisplatin in vitro; however, there have been no reports about whether blocking the PI3K/Akt cascade enhances the sensitivity to cisplatin in vivo. We investigated whether inhibition of PI3K increased the efficacy of cisplatin in an in vivo ovarian cancer model. Blocking the PI3K/Akt cascade with a PI3K inhibitor (wortmannin) increased the efficacy of cisplatin-induced inhibition of intraabdominal dissemination and production of ascites in athymic nude mice inoculated ip with the Caov-3 human ovarian cancer cell line. In addition, wortmannin increased the efficacy of cisplatin-induced apoptosis in tumors cells. There were no detectable side effects in mice treated with wortmannin. Moreover, the antitumor effect of cisplatin detected in mice inoculated with Caov-3 cells stably transfected with empty vector was significantly attenuated, compared with mice inoculated with Caov-3 cells stably transfected with a dominant-negative Akt, K179M-Akt. We confirmed that wortmannin blocked Akt phosphorylation and the downstream targets of the PI3K/Akt cascade, such as BAD (Bcl-2-associated death protein) and nuclear factor-kappaB in vivo by immunohistochemical staining and Western blotting. In accordance with the previously reported in vitro results, these in vivo results support the idea that combination therapy with cisplatin and a PI3K inhibitor would increase the therapeutic efficacy of cisplatin.