Adenocarcinomas exhibiting gastric differentiation represent a recently described and uncommon subtype of non-human papillomavirus (HPV) related cervical adenocarcinoma. They comprise a spectrum from a well differentiated variant (adenoma malignum/mucinous variant of minimal deviation adenocarcinoma) to a more poorly overtly malignant form, generally referred to as gastric-type adenocarcinoma. Rarely, such tumours have also been described as primary vaginal neoplasms. Gastric-type adenocarcinomas exhibit considerable morphological overlap with adenocarcinomas originating outside the female genital tract, especially mucinous adenocarcinomas arising in the pancreas and biliary tract. Moreover they often metastasize to unusual sites, such as the ovary and peritoneum/omentum, where they can be mistaken for metastatic adenocarcinomas from other, non-gynaecological sites. There is little information regarding the immunophenotype of gastric-type adenocarcinomas and knowledge of this is important to aid in the distinction from other adenocarcinomas. In this study, we undertook a detailed immunohistochemical analysis of a large series of cervical (n=45) and vaginal (n=2) gastric-type adenocarcinomas. Markers included were CK7, CK20, CDX2, CEA, CA125, CA19.9, p16, ER, PR, MUC6, PAX8, PAX2, p53, hepatocyte nuclear factor 1 beta (HNF1β), carbonic anhydrase IX (CAIX), HER2 and MMR proteins. All markers were classified as negative, focal (<50% of tumour cells positive) or diffuse (≥50% tumour cells positive) except for p53 (classified as “wild-type” or “mutation-type”), HER2 (scored using the College of American Pathologists guidelines for gastric carcinomas) and MMR proteins (categorised as retained or lost). There was positive staining with CK7 (47/47 – 45 diffuse, 2 focal), MUC6 (17/21 – 6 diffuse, 11 focal), CEA (25/31 – 12 diffuse, 13 focal), CAIX (20/24 – 8 diffuse, 12 focal), PAX8 (32/47 – 20 diffuse, 12 focal), CA125 (36/45 – 5 diffuse, 31 focal), CA19.9 (11/11 – 8 diffuse, 3 focal), HNF1β (13/14 – 12 diffuse, 1 focal), CDX2 (24/47 – 4 diffuse, 20 focal), CK20 (23/47 – 6 diffuse, 17 focal) and p16 (18/47 – 4 diffuse, 14 focal). Most cases were negative with ER (29/31), PR (10/11), PAX2 (18/19) and HER2 (25/26). p53 showed “wild-type” and ”mutation-type” staining in 27/46 and 19/46 cases respectively. MMR protein expression was retained in 19/20 cases with loss of MSH6 staining in one patient with Lynch syndrome. Molecular studies for HPV were undertaken in two tumours which exhibited diffuse” block-type” immunoreactivity with p16 and both were negative. This is the first detailed immunohistochemical study of a large series of gastric-type adenocarcinomas of the lower female genital tract. Our results indicate immunophenotypic overlap with pancreaticobiliary adenocarcinomas but suggest that PAX8 immunoreactivity may be especially useful in distinguishing gastric-type adenocarcinomas from pancreaticobiliary and other non-gynaecological adenocarcinomas which are usually negative. Diffuse “block-type”p16 immunoreactivity ...