The aim of the study was to investigate the possibility of predicting the histologic type of different breast cancers by semi-quantitative screening of cytologic criteria. Cytologic material was obtained by the aspiration technique. The frequency of ten cytologic criteria was correlated to five histologic types of breast cancer. Hitherto, it has not been possible to define histologic types of breast carcinomas on the basis of cytologic criteria alone. While the occurrence of cellularity, intercellular cohesion, foam cells and necroses often varies from slide to slide within one particular carcinoma, other features such as nuclear diameter, polymorphism, anisonucleosis, apocrine carcinoma cells, and enlarged nucleoli are more or less uniformly distributed. The application of these cytologic criteria thus uniformly represented in a tumour may prove to be a more reliable method for classifying breast carcinomas than the use of histologic growth patterns which are often inhomogenous within each individual neoplasm.