We describe a common, but hitherto not well described, reactive change of the endocervical surface epithelium, commonly seen in association with endometrial carcinoma, and which we term 'atypical reactive proliferation'. This lesion, especially when florid, has the potential to be misinterpreted as a manifestation of a stage 2A endometrial cancer (endocervical glandular involvement). We examined the cervical sections in 80 consecutive hysterectomy specimens of endometrial cancer. In 22 cases (27.5%), there was cervical involvement by tumour and these cases were excluded from further analysis. Of the remaining cases, atypical reactive proliferation involved the endocervical surface in 40 of 58 (69%) cases, although the degree of abnormality varied widely between individual cases. Histological features characteristic of atypical reactive proliferation (not all features were present in each case) included nuclear stratification and multilayering with short micropapillary processes, squamoid change, hobnail cells and mild cytological atypia. Other features present in some cases were surface erosion, clearing of the cytoplasm, fibrin deposition, an inflammatory cell infiltrate and fibrosis of the subepithelial tissue. In 20 control cases, comprising hysterectomy specimens for benign conditions, similar changes were not seen. Vimentin immunohistochemistry was undertaken in eight cases in which atypical reactive proliferation was particularly florid. Five cases were completely negative and three exhibited very focal positivity. Atypical reactive proliferation involving the endocervical surface is commonly seen in association with endometrial cancer and has the potential to be misinterpreted as endocervical involvement by tumour. Although this could represent a reactive change associated with the presence of an endometrial cancer, we feel atypical reactive proliferation is most likely a reactive/reparative response to recent endometrial biopsy or curettage. The vimentin-negative immunophenotype may be of value in cases where the uterine carcinoma is endometrioid in type as these neoplasms are generally vimentin positive.