Nucleolar organiser regions (NORs) were counted in the nuclei of both epithelial and stromal cells in cases of endometrial hyperplasia and carcinoma. The scores for each case were expressed as a ratio. No significant difference was found between the ratios for cystic hyperplasia and hyperplasia with architectural atypia, confirming the view that these are morphological variants of the same benign hyperplastic process. The ratios for hyperplasia with cytological atypia and adenocarcinoma differed significantly from the benign hyperplasias. This adds support to the view that hyperplasia with cytological atypia should be considered a neoplastic condition.