Immunological phenotyping of mononuclear cell suspensions from bone marrow and/or peripheral blood from 2341 patients (6140 examinations) suspected of malignant haematological diseases was performed during a 10-year period. The cells were labelled with a panel of monoclonal antibodies and subjected to flowcytometry. The results showed a clear distinction between acute lymphoblastic leukaemia and acute myeloid leukaemia, and a new group of acute hybrid leukaemia (3.3% of all acute leukaemia cases) was established. During the 10-year period immunological phenotyping has changed from being a research method to a widely employed method for cell characterisation early in the course of malignant haematological diseases.