A histochemical study revealed focal intracytoplasmic mucin within 50% of a series of adenocarcinomas arising within the endometrium. Seventeen percent of the adenocarcinomas studied contained intracytoplasmic O-acetylated sialomucins, as demonstrated by the PB/KOH/PAS technique. The presence of O-acetylated sialomucin, indicating enteric or intestinal differentiation, has not previously been demonstrated within endometrial adenocarcinomas, although it is well described in endocervical adenocarcinomas and in mucinous tumors of the ovary. In none of the cases in which enteric-type mucins were identified was there morphological evidence of intestinal differentiation in the form of goblet cells or Paneth cells. No correlation was found between the presence of enteric mucins and the grade of the adenocarcinoma. The results of the study provide further confirmation of the ability of the epithelial lining of the female genital tract to undergo various forms of metaplasia, along both Mullerian and non-Mullerian pathways. The demonstration of specific mucins is more sensitive than morphology in establishing enteric differentiation.