Bax protein, the proapoptotic member of Bcl-2 protein family, plays the key role in apoptosis pathway.The aim of the study was to assess the expression of Bax protein in breast cancer cells.Material and methods. Sixty-two breast cancer patients were included in the study. The control group encompassed 11 fibroadenoma patients. Single cells were isolated from defrosted samples and prepared for flow cytometry measurement.Results. Median expression of Bax protein in study group was 7.9% (range: 0-49.4%) and was significantly lower than in control (median expression 15.8%; range 4.9-30.9%; p=0.034). Expression of Bax correlated with expression of p53 and caspase-3 proteins (p<0,01, rank Spearman test). In patients under 70 years old and with positive estrogen receptors status the expression of Bax protein was significantly higher (p=0.03 and p=0.01 respectively).Conclusions. Lower expression of Bax protein in breast cancer cells may suggest the potential way of apoptosis avoidance of tumor cells. Correlations among Bax protein, p53 and caspase-3 are likely associated with active apoptotic mechanism in breast cancer cells expressing Bax protein. Further investigation with long time follow-up should be performed to establish the prognostic role of Bax protein expression in breast cancer patients.