Seventy-five breast samples including normal tissue, hyperplastic, metaplastic, atypical and neoplastic lesions were employed for the determination of interphasic Nucleolar Organizer Regions (NORs) modifications and Proliferating Cell Nuclear Antigen (PCNA) immunoreactivity. Interphase NORs were quantitatively and qualitatively modified in atypical lesions and breast carcinomas, whereas only modifications in the Ag-NORs count were found in benign samples. Our results investigated the nature of interphase NORs in the hope of finding a use for their evaluation in the diagnosis and biological clarification of breast epithelial atypia.