Objective: CD99 is a membraneous protein that is expressed widely among various soft tissue tumors. There is growing evidence that its expression in some carcinomas correlates with epithelial to mesenchymal transition (EMT) and is a poor prognostic factor. Its importance in the breast carcinoma remains unsettled. Method: The analysis was performed on breast cancer samples from 122 patients. Tumors were graded histologically according to the Nottingham system. CD99 expression was scored by grading system used for HER-2. Only cases showing grade (3+) were regarded as positive. Additionally, expression of estrogen (ER) and progesterone receptor (PR), HER-2, E-cadherin, Vimentin, Twist, Ki-67, c-myc, cyclin D1 and Topoisomerase 2alfa was performed. Expression of the CD99 was correlated with all these markers and with clinical outcome. Results: Expression of CD99 was observed in 11 patients and correlated significantly with negative PR status (p=0,01), higher histological grade (p=0,05), positive Twist (p=0,04) and Topoisomerase2alfa expression (p=0,04). There was also a trend toward higher frequency of cyclin D1 positivity (p= 0.09). No impact on prognosis for CD99 expression was found. Conclusion: Expression of CD99 correlates with high histological grade, negative progesterone receptor status and expression of certain EMT and proliferation markers, but it does not influence prognosis in breast carcinoma.