In CGB laboratory are investigated 500 of malignant breast tumors every year. We evaluate the morphology, grade of differentiation and phenotype properties-hormone receptors expression, overexpression and amplification of HER-2/neu gene. 2 % of carcinomas posses significantly morphologically and phenotypically heterogenic tumor population in one and the same tumor leasion. Results: The most often sign of phenotype heterogenity found is hormone receptors expression or overexpression of HER-2/neu gene. Often we see two morphologically different tumor populations coexisting. The most interesting cases from our practice are on the poster. Conclusion: As for now, studies about the origin of heterogenity inside the same leasions, did not find a single theory to resolve it. A possible answer gives the theory of tumor stem cells and the model of clonal evolution. Our observations demonstrate and may explain the practical reason of different phenotype properties found in core cut biopsy where one population may be held, against the heterogenic properties of the tumor shown when resected as a whole.