The observation of koilocyte-like features in the squamous epithelium of some endometrial adenoacanthomas prompted an investigation into a possible viral aetiology. These changes closely resemble those that occur in the ectocervical mucosa which are accepted as morphological evidence of human papillomavirus (HPV) infection. Sections of 87 hysterectomy specimens removed for endometrial carcinoma over 12 years, together with preoperative curettings, were reviewed for the presence of acanthomatous change and for appearances suggestive of HPV infection. The ages of the women ranged from 36 to 84 years, average age 62.6. Light microscopical examination showed koilocytosis, papillary formations, and intranuclear eosinophilic inclusions of both squamous and glandular epithelium in some tumours. Immunocytochemistry and DNA in situ hybridisation indicated the presence of HPV antigen in squamous and glandular cells, and perinuclear virus particles characteristic of HPV were seen on electron microscopical examination in those cells with nuclear inclusions. HPV probably infects endometrial adenocarcinomas directly from the cervix but it is unlikely that it has an aetiological role. It is possible, however, that in addition to being a "passenger," the virus may stimulate squamous metaplasia in some adenocarcinomas of the endometrium and may also exert some influence on their behaviour.