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There are no specific recommendations for evaluating the Ki-67 index in heterogeneous breast carcinomas. This study aimed to evaluate the applicability of currently accepted recommendations for Ki-67 evaluation in breast cancer in the context of intratumor heterogeneity. Twelve cases of heterogeneous breast carcinomas obtained from 110 patients were retrospectively studied. Ki-67 staining was performed according to protocols provided by the reagent manufacturer. Results for Ki-67 of the separate components in each tumor were obtained, described, and analyzed statistically using a paired t-test. Values of p < 0.05 were considered as statistically significant. SPSS software was used for statistical analysis. Results from the comparison of the Ki-67 index evaluation in each heterogeneous component of the studied tumors demonstrated no statistically significant difference of mean values t = 0.4802, p = 0.6405. The anticipation of an average Ki-67 score in the evaluated cases would have changed the molecular subtype from Luminal B to Luminal A (due to the Ki-67 index below 14%) in two of the cases. Heterogeneous tumors had a different Ki-67 index in their separate components. Our observations suggest that Ki-67 in heterogeneous breast carcinoma is evaluated and reported separately for the distinguishable tumor components.