Uterine serous carcinomas typically have a characteristic morphology (papillary architecture, high-grade nuclei) and immunoprofile (diffuse/strong p53 expression, loss of hormone receptor expression) that distinguish them from most endometrial endometrioid carcinomas. However, glandular variants of serous carcinoma can simulate Fédération Internationale de Gynécologie et d'Obstétrique (FIGO) grade 2 endometrioid carcinomas, and some serous carcinomas lack p53 expression and retain hormone receptor expression, making classification difficult. P16 expression patterns distinguish endometrioid carcinomas (patchy) from human papillomavirus (HPV)-related endocervical adenocarcinomas (diffuse/strong) but utility for distinction of serous carcinomas from endometrioid carcinomas and endocervical adenocarcinomas has not been evaluated in a large series. Immunohistochemical analysis of p16 expression was performed on 201 uterine and endocervical adenocarcinomas in hysterectomy specimens, including 49 serous carcinomas, 101 endometrial endometrioid carcinomas (44 FIGO grade 1, 40 FIGO grade 2, and 17 FIGO grade 3), and 51 HPV-related endocervical adenocarcinomas. All serous carcinomas demonstrated diffuse/moderate-strong p16 expression, with percentage of positive tumor cells ranging from 90% to 100% (mean/median: 95%/100%). In contrast, endometrial endometrioid carcinomas exhibited less diffuse and less intense expression, with percent of positive tumor cells ranging from 10% to 90% (mean/median: 38%/30%; staining intensity: variable). Similar to serous carcinomas, all endocervical adenocarcinomas exhibited diffuse/moderate-strong p16 expression, with percentage of positive tumor cells ranging from 90% to 100% (mean/median: 94%/90%). P16 can serve as an additional diagnostic marker, used as part of an immunohistochemical panel, including p53 and hormone receptors, for distinction of uterine serous carcinomas from endometrioid carcinomas. Distinction of serous carcinomas from endocervical adenocarcinomas (HPV-related type), both of which share diffuse p16 expression and frequently lack hormone receptor expression, relies on morphology and diffuse/strong p53 expression in the former and detection of HPV in the latter.