Cell atypia in breast fine needle aspiration (FNA) can introduce some diagnostic difficulties. Molecules reflecting proliferative cell potential, such as Ki‐67 and p27Kip1, can help in recognizing the true biological nature of a cell. Thus, the objective of the study was to analyze the difference in Ki‐67 and p27Kip1 cell immunoexpression in breast FNA specimens between fibroadenomas, fibrocystic changes (FCC) with atypia, and breast carcinoma. Microscopic analyses of cell cytomorphology and Ki‐67 and p27Kip1 breast cell immunoexpression were done after standard Pappenheim and immunocytochemical staining (labeled streptavidin‐biotin, LSAB) method in autostainer DakoCytomation TechMate™. The study included 50 patients with breast carcinoma, 20 patients with fibroadenoma, and 20 patients with FCC with atypia. High Ki‐67 and low or absent p27Kip1 were found in most patients with breast carcinoma, while majority of FCC with atypia were characterized by low Ki‐67 and moderate to high p27Kip1 cell immunoexpression. Majority of fibroadenomas were associated with low Ki‐67 and low to moderate p27Kip1 cell immunoexpression indicating progressive decrease in cell cycle inhibition, but still not so high proliferative activity as in carcinoma. However, although statistically significant difference for Ki‐67 and p27Kip1 was found between breast lesions in our study, the large ranges observed for each marker make them essentially useless for better cytological diagnosis in a single case. Regarding their opposite role in cell cycle, inverse correlation of Ki‐67 and p27Kip1 was noticed. Poorly differentiated carcinoma cells had mostly high Ki‐67 andlow p27Kip1 cell immunoexpression. Diagn. Cytopathol. 2011;39:333–340. © 2010 Wiley‐Liss, Inc.