Neoplasms that are composed focally, predominantly, or exclusively of osteoclast-like giant cells admixed with variably pleomorphic mononuclear cells have been described in a wide variety of organs. In this report, we describe the case of a 76-year-old woman with an 8-cm tumor that appeared to be localized to the ovary, that was composed predominantly of diffusely distributed, bland-appearing osteoclast-like giant cells admixed with pleomorphic mononuclear cells, and that was not associated with an ovarian cystic neoplasm. Hemorrhage, large zones of necrosis, and a high mitotic index were the other characteristics of the tumor. Immunohistochemically, the mononuclear cells were strongly positive for vimentin and proliferating cell nuclear antigen and were negative for keratin AE 1/3, CAM 5.2, cytokeratin 7, epithelial membrane antigen, beta-human chorionic gonadotropin, desmin, smooth muscle actin, p53, leukocyte common antigen, S-100, inhibin, alpha-1-antichymotrypsin, and CD68. The osteoclast-like giant cells displayed immunoreactivity for CD68, vimentin, alpha-1-antichymotrypsin, and leukocyte common antigen only. Ultrastructurally, rare intercellular junctions were present between mononuclear cells, suggestive of an epithelial histogenesis. Less than a dozen ovarian lesions with the "giant cell" designation have been described, and most of these cases are thought to be analogous to the "sarcoma-like" nodules or other such lesions that have a well-known association with ovarian cystic neoplasms. Our case, in contrast, did not have an easily identifiable epithelial component and demonstrated both an infiltrative border and vascular invasion. This is, to the authors' knowledge, the first detailed clinicopathologic description of such a case as an ovarian lesion.