Undifferentiated carcinoma of the endometrium (UCAe) is an aggressive, underrecognized high-grade carcinoma that can occur either in pure form or in conjunction with low-grade endometrioid adenocarcinoma (i.e. dedifferentiated carcinoma). The typical solid growth pattern of UCAe can create a diagnostic dilemma as it is frequently misinterpreted as the solid component of an endometrial carcinoma or as a sarcoma. In addition, the high nuclear:cytoplasmic ratio, high mitotic index, and geographic necrosis are reminiscent of basal-like carcinoma of breast (BLCB). This study was undertaken to determine the role of a selected group of immunomarkers in the distinction of UCAe from other endometrial carcinomas, and assess the expression of DNA mismatch repair proteins, and surrogate BLCB immunomarkers in this type of tumor. Cases of UCAe were stained with antibodies against keratin cocktail, CK8/18, PAX-8, and estrogen receptor: 35 cases; progesterone receptor and Her-2/neu: 33 cases; CD44, e-cadherin, p16, and p53: 32 cases; and CK5/6, EGFR, and c-Kit: 18 cases. In addition, mismatch repair protein markers MLH1, MSH2, MSH6, and PMS2 were performed in 34 cases. We found that PAX-8 expression was lost in most cases (83%). In addition, estrogen and progesterone receptors were negative in 83% and 82% of cases, respectively. Seventy-seven percent of cases were positive for keratin cocktail and keratin 8/18, whereas only 11% of cases were positive for keratin 5/6. p16 was diffusely positive in 34% of cases, whereas p53 was expressed in >75% of the tumor cells in 31% of cases. MLH1 and PMS2 were concurrently lost in 50% of cases, whereas MSH2 and MSH6 were lost in 1 case (3%). E-cadherin and CD44 were completely lost in 50% of cases, whereas Her-2/neu was negative in all cases. EGFR was negative in 67% of cases, whereas 22% of cases showed diffuse membranous staining for this marker. UCAe is a high-grade carcinoma of Müllerian origin which tends to be negative for PAX-8. The loss of this marker appears to be a more reliable discriminator than the loss of keratin expression in the differential diagnosis with endometrioid carcinoma or serous carcinoma. UCAe tends to be diffusely positive for p53, but patchy positive for p16. Although UCAe appears to share not only some histologic features with BLCB, but also some of its immunohistochemical features (loss of estrogen receptor, progesterone receptor, and Her-2/neu, a tendency to loose e-cadherin and to express CD44), UCAe appears not to be related to BLCB because it usually lacks the expression EGFR, CK5/6, and c-Kit.