The histological distinction between endometrial and endocervical adenocarcinoma may present diagnostic difficulties. Several reports have suggested that differences in the histochemical staining of the tumours may indicate their histogenesis. In this paper we have compared the staining properties of 20 endocervical adenocarcinomas (14 invasive and six in situ) with 12 cases of endometrial adenocarcinoma. Unequivocal examples were used where the site of origin was not in doubt. Sections were stained using a monoclonal antibody to carcinoembryonic antigen (CEA), alcian blue at pH 1.0 and pH 2.5, PAS-diastase and toluidine blue. The results were graded by two pathologists and results compared. Three poorly differentiated invasive endocervical tumours were completely negative for CEA. Seventeen out of the remaining 20 endocervical tumours and 7/12 endometrial adenocarcinomas showed a non-specific apical staining pattern. This also occurred in normal and dysplastic endocervical glands. Intense intracytoplasmic staining for CEA was found in 7/14 invasive and 3/6 in situ endocervical adenocarcinomas. Normal endocervix never showed this staining reaction. However, 4/12 endometrial tumours showed a similar pattern. Mucin stains showed no specific staining pattern for either tumour. In an individual case where the site of origin is in doubt, histochemical methods are of little value in confidently resolving the problem.