Giant cell carcinoma (GCC) is a highly aggressive variant of sarcomatoid carcinoma of the lung. To date, however, there have been no reported cases of ovarian carcinoma mainly composed of GCC. Herein is reported the case of a 54-year-old Japanese woman with an undifferentiated ovarian carcinoma producing granulocyte colony-stimulating factor (G-CSF) and an inflammatory cytokine. Histologically, the tumor was composed of cohesive nests or discohesive pleomorphic mononucleated or multinucleated tumor giant cells, accompanied by inflammatory cell infiltration and emperipolesis. Immunohistochemically, the tumor cells were focally positive for epithelial membrane antigen and cytokeratin 7. Clinically, after the initial surgery, the tumor had rapid regrowth along with the production of G-CSF and an inflammatory cytokine. Adjuvant chemotherapy was administered but induced severe heart failure and severe neutropenia, probably due to the presence of hypercytokinemia and excess G-CSF. Upon the appearance of these fatal side-effects the chemotherapy was immediately discontinued and replaced with radiotherapy. The recognition of this type of ovarian tumor is important for clinical management, because adjuvant chemotherapy is the standard treatment for clinical management of epithelial ovarian cancer.