We describe a series of unusual endometrioid carcinomas (ECs) of the uterine corpus characterized in significant part by cords of epithelioid cells, spindle cells, and a hyalinized stroma that sometimes formed osteoid. These features, particularly when prominent, produced an appearance strikingly different from that of conventional EC, sometimes resulting in problems in differential diagnosis, especially with a malignant mullerian mixed tumor (carcinosarcoma). The 31 patients ranged in age from 25 to 83 years (mean, 52 years). The proportion within each stage were as follows: stage Ia, 9.7%, stage Ib, 45.2%, stage Ic, 9.7%, stage IIb, 16.1%, stage IIIc 3.2%, and stage IV, 3.2%. In 4 patients (12.9%), staging information was not available. On microscopic examination, typical EC, which accounted for 10% to 90% of the tumor, was admixed in 90% of cases with cords of epithelioid or spindle cells within a hyalinized stroma. In 3 cases, the tumor contained cords of cells without a hyalinized stroma. Areas with a diffuse growth of fusiform cells suggesting endometrial stromal cells were also occasionally seen in minor amounts. Seventy percent of the tumors exhibited squamous differentiation, and in 50% of the tumors there was a background of endometrial hyperplasia. Two thirds of the tumors were grade 2 and the remainder were grade 1. Vascular space invasion was identified in seven tumors. On immunohistochemical analysis, the typical EC component was strongly positive for keratin, whereas the keratin staining was more focal and variable in the epithelial cells in the cords. Muscle markers (desmin, actin), CD10, and inhibin were negative in the latter. Overexpression of p53 was found in only 1 case. Eighty-three percent of the patients were alive with no evidence of disease on follow-up (range, 2-115 months; mean, 34.4 months). The clinical features, including a typically low stage and generally good prognosis, and histologic findings are different from those of malignant mullerian mixed tumors that are characterized by both high-grade carcinomatous and sarcomatous components and an aggressive clinical course. Confusion with other neoplasms, particularly those with sex cord-like growth, such as uterine tumors resembling ovarian sex cord tumors and epithelioid smooth muscle tumors, may also arise. We refer to tumors with the features described herein as "corded and hyalinized endometrioid carcinomas," a designation that reflects their two most striking and consistent features. Corded and hyalinized endometrioid carcinomas are yet another example of the protean phenotype of endometrioid adenocarcinomas of the female genital tract that has been appreciated only in the last two decades.