Malignant mixed müllerian tumors are very rare neoplasms of the fallopian tube, and treatment is not well defined. A case of malignant mixed müllerian tumor of the tube concomitant with a primary serous carcinoma of the ovary is reported. It was unclear if there were two distinct neoplasms in the same patient, or if it was a single tumor with a sarcomatous fallopian conversion of the serous component, as described for some recurrent ovarian carcinomas. Chemotherapy for ovarian carcinoma with intraperitoneal metastasis was performed, with about a three-year interval-free period of disease, as could be expected for ovarian carcinomas at the same stage. Such coexistence of these two tumors does not afford adequate staging of the malignancy. Therapy for the very rare cases similar to the one reported here needs to be improved.