Modern diagnostics of hematological diseases is a combination of different laboratory techni- ques. Flow cytometry is an indispensable tool for the diagnosis of leukemia and lymphoma used in addition to classical cytology, cytochemistry, cytogenetics and molecular biology. The advantage of flow cytometry for diagnosis in hematology is a multiparameter assessment of antigens on a single cell level. Specifically in cases of leukemia flow cytomerty allows the determination lineage and maturation status of the malignant cells, and so the classification of the subtype of leukemia, identification of aberrant antigen expression and makes possible further detection of minimal residual disease. This article includes the immunological classification of acute leukemias, shows the examples of different monoclonal antibody panels that can be used for leukemia diagnostics and presents a selection of case reports taken from the routine work in our Department of Hematology. This article could be of practical help for people com- mencing immunophenotyping of acute leukemias.