Choriocarcinoma has been reported in association with endometrial carcinoma and as a metaplastic change in multiple carcinomas, including liver, urinary bladder, lung, and the gastrointestinal tract. We report choriocarcinoma in conjunction with a carcinosarcoma (also called malignant mullerian mixed tumor) in a 71-year-old woman whose hysterectomy specimen revealed two polypoid lesions of the endometrium, one arising from the anterior endometrium and one arising from the posterior endometrium. Histologic examination revealed three histologic patterns. The anterior endometrial lesion showed a FIGO grade 2 endometrioid endometrial adenocarcinoma. The posterior endometrial lesion showed a carcinosarcoma composed of a high-grade adenocarcinoma and scant homologous stromal sarcoma. In addition, a choriocarcinoma was identified intermixed with the adenocarcinoma. The syncytiocytotrophoblasts and cytotrophoblasts stained strongly with β human chorionic gonadotropin (β-hCG) and human placental lactogen (hPL). The patient's β-hCG levels on postoperative days 14, 27, and 42 were 283, 32, and 7 mIU/mL, respectively. This unusual case suggests the importance of identifying the choriocarcinomatous component, since the serum β-hCG can serve as a marker of tumor recurrence postoperatively.