Determining the origin of uterine adenocarcinomas can be difficult in biopsy and curettage specimens because the morphologic spectrum of endocervical and endometrial adenocarcinomas overlaps. In addition, in hysterectomy specimens the primary site is often equivocal for tumors that involve predominantly the lower uterine segment and endocervix and lack identifiable precursor lesions. We assessed the value of immunohistochemistry for estrogen and progesterone receptors and in situ hybridization for human papillomavirus DNA detection in making this clinically relevant distinction. We evaluated a set of 48 adenocarcinomas of unequivocal origin (24 endocervical carcinomas and 24 endometrial endometrioid carcinomas without cervical extension) and then tested seven lower uterine segment/endocervical carcinomas of equivocal origin to determine whether patterns established in the initial set would permit definitive assignment of primary site for the equivocal set. Only one (4.2%) of 24 endocervical carcinomas was positive for both estrogen receptor and progesterone receptor, whereas 18 (75%) of 24 endometrial carcinomas were positive for estrogen receptor and 23 (95.8%) of 24 endometrial carcinomas were positive for progesterone receptor (p <0.001, chi2 test). Human papillomavirus DNA was detected in 16 (66.7%) of 24 endocervical carcinomas and in none of 24 endometrial carcinomas (p <0.001, chi2 test). Of the seven tumors of equivocal origin, five could be definitively classified as either endocervical or endometrial in origin based on their demonstration of a characteristic profile with these assays (either estrogen receptor/progesterone receptor-negative/human papillomavirus-positive, consistent with endocervical origin or estrogen receptor/progesterone receptor-positive/human papillomavirus-negative, consistent with endometrial origin). We conclude that hormone receptor immunohistochemistry and human papillomavirus in situ hybridization are useful for distinguishing endocervical and endometrial adenocarcinomas. The clinical utility of these techniques should be evaluated in studies that include curettage and biopsy specimens.