Background: The expression level of several matrix metalloproteinases (MMPs), including MMP-2 and MMP-9, in ovarian cancer cells is directly associated with their invasive and metastatic potentials. MMP-9 is also expressed in stromal cells adjacent to the tumor. To investigate the contribution of MMP-9 expression in stromal cells to ovarian tumor growth, we examined angiogenesis and progressive growth of human ovarian cancer cells implanted into mice with and without the MMP-9 gene. Methods: Human ovarian cancer cells SKOV3.ip1 and HEY-A8 were implanted into the peritoneal cavities of nude mice that lacked the gene for MMP-9 (MMP-9^−/−) or were wild type for MMP-9 (MMP-9^+/+) (10 mice of each genotype per cell line). Tumor incidence, tumor size, and volume of ascites fluid were recorded for each mouse at 30 and 45 days after HEY-A8 and SKOV3.ip1 cell injections, respectively. Blood vessel density and macrophage infiltration into the lesions were analyzed in excised tumors by immunohistochemistry and double immunofluorescence. Tumor growth was also studied in MMP-9^−/− nude mice that had been reconstituted with spleen cells collected from either MMP-9^+/+ or MMP-9^−/− nude mice. All statistical tests were two-sided. Results: HEY-A8 cells expressed high levels of MMP-9, and SKOV3.ip1 cells expressed low levels. Nevertheless, tumor incidence and growth were statistically significantly lower in MMP-9^−/− mice than in MMP-9^+/+ mice injected with cells from either line (for tumor size, P = .006 and .042 for HEY-A8 and SKOV3.ip1 cells, respectively). Compared with MMP-9^+/+ mice injected with human ovarian cancer cells, MMP-9^−/− mice injected with human ovarian cancer cells displayed decreased microvessel density and decreased macrophage infiltration into the lesions. Compared with MMP-9^−/− mice that received spleen cells (a rich source of macrophages) from MMP-9^−/− mice, those that received spleen cells from MMP-9^+/+ mice before cancer cell injections displayed increased angiogenesis and tumorigenicity of the cancer cells. The growing tumors contained MMP-9-expressing macrophages. Conclusion: Host-derived MMP-9 expression, most likely in tumor-infiltrating macrophages, appears to play a critical role in angiogenesis and progressive growth of human ovarian tumors in mice.