Pathogenetically, endometrioid adenocarcinomas of the endometrium are associated with hyperestrogenism and serous papillary carcinomas with alterations of p53. The etiology of primary endometrial squamous cell carcinoma (ESCC), however, is speculative. The purpose of this study was to evaluate the role of p14, p16, p53, cyclin D1, steroid hormone receptors, and human papillomaviruses (HPV) infection in the pathogenesis of primary endometrial squamous cell carcinoma. The expression of p16, p14, p53, cyclin D1, and steroid hormone receptors (estrogen, progesterone, and androgen) was examined immunohistochemically in 8 primary ESCCs. HPV analysis was performed using general primers and HPV typing. The median age of the patients was 62.1 years. Four cases showed positive nuclear and cytoplasmic p16 staining in an insular pattern, and 1 case nuclear positivity for p53 and estrogen receptors, respectively. Four of 8 cases were positive for progesterone receptor analysis and cyclin D1. All cases were negative for p14 and androgen receptor staining. All but one case were negative for HPV analysis. Five patients were alive with and without evidence of disease after a mean follow-up of 6.1 years. The results of this study suggest that alterations of the p16 pathway may play an etiologic role in at least a proportion of the ESCC, but without any association to HPV infection. Factors known to play a pathogenetic role in types 1 and 2 of endometrial carcinomas are not associated with primary ESCC. However, prognostically, ESCCs are more related to type 1 cancers.