Cervical high-grade squamous intraepithelial lesions (HSILs) are typically characterised by a proliferation of immature basaloid cells with relatively uniform hyperchromatic nuclei. In this report we describe 19 cases of HSIL exhibiting focal but very marked nuclear atypia often associated with multinucleation ('pleomorphic HSIL'). The bizarre cytological changes mainly involved the basal epithelium particularly in endocervical crypts where the neoplastic cells undermined the native glandular epithelial cells. Superficially invasive squamous cell carcinoma (SISCCA) was present in three cases (16%) and while this was more common than in a comparative series of 40 'conventional' HSIL excision specimens (5%), the difference was not statistically significant. All three invasive cases demonstrated additional histological features that have been associated with increased risk of SISCAA (expansile crypt involvement by HSIL, luminal necrosis, and/or intraepithelial squamous maturation), and the invasive foci were associated microanatomically with conventional-type rather than pleomorphic HSIL. The bizarre cells expressed p16 and p63 proteins but usually lacked mitotic activity and showed less Ki-67 labelling than adjacent conventional HSIL. These findings suggest that pleomorphic epithelial changes in HSIL do not necessarily indicate more aggressive biological behaviour and may, in some cases, represent a degenerative phenomenon.