Background: Dysregulated miRNA expression has been found in nearly all the stages of cancer process compared to normal tissue. Certain miRNAs expression level is closely related to the histopathological parameters and molecular subtypes of breast cancer (BC) as well as the response to treatment and prognosis.Methods: This prospective research has evaluated the level of expression of 5 miRNAs (miR-124a, miR-137, miR-34a, miR-155, miR-373) in the primary tumor before and after chemotherapy in patients with operable BC. The patients received 2-4 cycles neoadjuvant chemotherapy (NACT) and then underwent surgery – mastectomy or breast conservation with axillary lymph node dissection. Assessment of all mRNAs was analyzed in selected pathology specimens obtained during the biopsy and the targeted area taken by the surgery. Results: A total of 34 patients were treated between February 2016 and December 2017 with median age of 53 years (23 – 75). 70.6% were HR-positive and 29.4% were HR-negative. AntioncomiR-124a expression was mostly downregulated in primary BC tissue (p=0.001). The restored expression of miR-124 (p=0.004) was observed after chemotherapy having reached the indistinguishable level from non-tumor tissue. The dynamics of miR-373 expression considered to be oncomiR was a 7.7-fold decrease level in the primary tumor with continued suppress of expression compared to non-tumor tissue (p=0.006). The expression levels of miR-34a, miR-137, miR-155 in the pathology specimens did not show the differences compared to the surrounding non-tumor tissue (p>0.05). The difference in the dynamics of the two suppressor miRNAs (miR-124a and miR-137) expression in HR-positive BC was identified. While the expression of miR-124a in the tumor tissue increases after NACT (p=0.021), the expression of miR-137 continues to decrease (p=0.041) and does not differ from the levels of non-tumor tissue (p=0.146). There was no relation between the expression of the 5 studied miRNAs with histopathological differentiation of the primary tumor and the pathomorphological response after NACT. A Cox proportional hazards model did not confirm the relationship between miRNAs expression level neither in primary tumor nor in clinicopathological BC characteristics.Conclusions: The differential regulatory effects of miRNA cannot be explained by the differences in expression alone as paradoxical behavior of miR-373 shows. Taking into account the heterogeneity of the breast tumor, the detected inconsistency of changes in the expression of two suppressor miRNAs (miR-124a and miR-137) can be explained by their different regulatory functions. An accurate picture of the functioning of microRNA requires further research due to its dynamic nature.