CD45 cell surface antigen is a transmembrane protein with tyrosine phosphatase activity, expressed by all nucleated cells of hematopoietic origin, except erythrocytes and platelets. Monoclonal antibodies directed against CD45 represent irreplaceable tool in differential diagnosis of hematologic and other, non-hematologic low differentiated malignancies, primarily in cases of: extranodal lymphomas, non-hematologic malignancies with nodal or bone marrow localization or their metastases in mentioned sites. As cell surface immunophenotype marker, CD45 is of great value in differentiation of lymphoproliferative diseases subtypes. By flow cytometry, based on CD45 expression, the malignant cell population is being identified and that fact is used in, not only diagnosis, but also in detection of minimal residual disease, especially in cases of CD45 negative acute leukemias. Incidence of childhood CD45 negative acute lymphoblastic leukemias (ALL) is about 10%. Children diagnosed with low CD45 expression ALL generally have better prognosis than those with high CD45 expression, especially when cut-off value for CD45 expression is set on 90%. We have analyzed CD45 expression by flow cytometry in 28 consecutive patients diagnosed with ALL in our institution during a 5-year period. Among these patients 7.1% were CD45 negative. A positive correlation between CD45 and CD20 expression was found, and a negative correlation between CD45 and CD34. In our group of patients, CD45 expression did not have any influence on survival.