The immunohistochemical expression pattern of p16 in biopsy samples has been useful as part of a panel to distinguish adenocarcinomas arising from the endometrium from those arising from the endocervix. However, no information is available on the expression of p16 in uterine serous carcinoma (USC) or ovarian high-grade serous carcinoma that could be used for diagnostic purposes. Here, we retrospectively analyzed the immunohistochemical expression of p16 in 11 cases of USC (5 pure and 6 mixed with endometrioid adenocarcinoma) and 10 cases of ovarian high-grade serous carcinoma and compared p16 expression with that of p53 in the same samples. p16 was strongly expressed by 100% of tumor cells in all 11 uterine specimens and in 5 of the 10 ovarian specimens; of the other 5 ovarian specimens, 4 showed strong positivity in 20% to 80% of tumor cells, and 1 case showed only weak expression. Positivity for p53 was strong and diffuse (100% of tumor cells) in 5 uterine tumors and in 3 ovarian tumors. p53 expression in 6 of the uterine specimens and 7 of the ovarian specimens was present in fewer tumor cells, of weak intensity, or both. We also performed human papilloma virus (HPV) DNA in situ hybridization in 4 uterine pure serous carcinomas; all 4 were negative. The negative results were confirmed by reverse transcriptase in situ polymerase chain reaction. We conclude that p16, owing to its diffuse expression in USC, should not be interpreted as indicating cervical origin or HPV-induced carcinogenesis; however, p16 may be a better marker (albeit unspecific) than p53 for identifying USC. The overexpression of p16 in USC is unrelated to HPV. Further studies are necessary to determine whether p16 expression is useful in the differential diagnosis of ovarian high-grade serous carcinoma.