Malignant mixed mesodermal tumor is a rare tumor of the ovary and its histogenesis is controversial. We report the case of an ovarian tumor that seemed to be a pure carcinoma and recurred as a carcinosarcoma, and suggest a possible histogenesis for this kind of tumor. The patient was a 62-year-old Japanese woman. The primary tumor was confined to the right ovary and was a histologically poorly differentiated endometrioid adenocarcinoma with focal squamous differentiation. The tumor recurred as peritoneal dissemination 9 months later showing a histological appearance of carcinosarcoma of heterologous type. The recurrent tumor also contained intermingled foci of similar histology as the primary tumor. The carcinomatous component of the recurrent tumor showed more obvious differentiation to adenocarcinoma with increased expression of epithelial markers compared to the primary tumor. Epithelial membrane antigen was positive also in a few cells of the sarcomatous component, which implies that this tumor had features of metaplastic carcinoma. The DNA ploidy pattern of the primary ovarian tumor was diploid, while an additional aneuploid subpopulation appeared in the recurrent tumor. These findings suggest the possible histogenesis of carcinosarcoma of the ovary as progression and clonal evolution of endometrioid adenocarcinoma.