Over recent years, the association between human papillomavirus (HPV) and squamous cell carcinoma of the cervix and its precursors has been well established, largely as a result of advances in the techniques of molecular biology. A similar association between HPV and invasive adenocarcinoma of the cervix has also been recently demonstrated; however, little work has been published on the relationship between the precursor lesion, adenocarcinoma in situ (ACIS), and HPV. We have therefore undertaken an in situ hybridization study of 22 cases of known cervical ACIS using probes for HPV messenger RNA expression. Seventeen of these had residual ACIS in the blocks studied, 15 of which (88.6%) expressed HPV messenger RNA. Five cases were positive for HPV 16 and 10 cases for HPV 18. Early invasive adenocarcinoma was present with ACIS in three cases and one case had early invasion plus cervical intraepithelial neoplasia (CIN III). These invasive lesions showed a similar type and degree of HPV expression as the in situ component. One section had only residual CIN III, which was positive for HPV type 18. Four cases had only minor glandular atypias (less than ACIS) in the studied sections, and all four of these were negative for HPV expression. None of the normal endocervical epithelia in any of the sections were positive by this technique. In summary, the finding of HPV messenger RNA expression in nearly 90% of cervical ACIS supports a possible role for these viruses in the pathogenesis of glandular neoplasms of the uterine cervix.