Plasminogen activator inhibitor-1 (PAI-1), which can be produced by host and tumor cells in the tumor microenvironment, is intimately involved in tumor progression. In the present study, to pursue the possibility that PAI-1 could be a therapeutic target in the management of malignancy, SK-216, a specific PAI-1 inhibitor, was orally administered to wild-type mice that were subcutaneously implanted or intravenously injected with either PAI-1–secreting Lewis lung carcinoma (LLC) or PAI-1–nonsecreting B16 melanoma cells. The systemic administration of SK-216 was found to reduce the size of subcutaneous tumors and the extent of metastases, regardless of PAI-1 secretion levels from the tumor cells. SK-216 also reduced the extent of angiogenesis in the tumors and inhibited VEGF-induced migration and tube formation by human umbilical vein endothelial cells in vitro. Then, to determine whether host or tumor PAI-1 was more crucial in tumor progression and angiogenesis, PAI-1–deficient or wild-type mice were subcutaneously implanted or intravenously injected with LLC or PAI-1 knockdown LLC cells. Tumor progression was shown to be controlled by the presence of host PAI-1 and not affected by the PAI-1 levels in the tumors. Similarly, host PAI-1 played a more crucial role in tumor angiogenesis than did tumor PAI-1. These observations suggest that regardless of the PAI-1 levels in the tumor, the systemic administration of SK-216 exerts an antitumor effect through its interaction with host PAI-1. This antitumor effect might be mediated by the antiangiogenic properties of SK-216. Mol Cancer Ther; 12(11); 2378–88. ©2013 AACR.