Low-grade endometrioid carcinomas, either of the endometrium or the ovaries, usually have an excellent prognosis. The association of this type of tumor with undifferentiated carcinoma is rare. In this study, we present the clinicopathologic features of 25 such cases. The age of the patients ranged from 30 to 82 years (median, 51 years). At presentation, the patients had either vaginal bleeding or pelvic pain. The endometrioid carcinoma involved the endometrium in 14 cases, the endometrium and 1 or both ovaries in 9 cases, and the ovaries in 2 cases. Undifferentiated carcinoma associated with low-grade endometrioid carcinoma was found at presentation in 19 grade 1 or 2 endometrioid carcinomas: 15 in the endometrium and 5 in the ovary. In one of these cases, undifferentiated carcinoma was found in the endometrium and the ovary. Undifferentiated carcinoma was found after resection of low-grade endometrioid carcinoma in six cases, involving the retroperitoneum, pelvis, vagina, or liver. The undifferentiated carcinoma was composed exclusively of diffuse sheets and solid nests of epithelial cells in l0 cases. Epithelial cells with isolated foci of keratinization were seen in nine cases and rhabdoid cells in a myxoid background in six cases. Twenty-four patients were treated with total abdominal hysterectomy and with bilateral salpingo-oophorectomy. Twenty-two patients received additional therapy as follows: chemotherapy (), radiotherapy (), and tamoxifen (). Follow-up showed that 15 patients died of disease in 1 to 60 months (median, 6 months), and 5 patients are alive with progressive disease with a follow-up between 6 and 8 months; 1 patient is alive with no evidence of disease at 104 months. In four cases, the diagnosis was made recently, with short follow-ups of 3 and 4 months. Foci of undifferentiated carcinoma may be confused with solid endometrioid adenocarcinoma erroneously leading to the diagnosis of a grade 3 or a significantly less aggressive grade 2 endometrioid carcinoma. The recognition of undifferentiated carcinoma in an otherwise low-grade endometrioid adenocarcinoma is extremely important because it indicates aggressive behavior. In asynchronous cases, being aware of this association can explain the absence of a second primary.