Flow cytometry is an automated method for measuring features of single cells in suspension. Many parameters may be measured simultaneously at high speed, objectively and with good statistics. The method has been increasingly used in the context of medical diagnosis and prognosis over the last ten years. Clinical-chemical departments are major users of flow cytometry for differential cell counts of leukocyte populations in blood by light scatter measurements. For immunophenotyping of acute leukaemias, flow cytometry is used routinely in all university hospitals in Norway. The method is also used to classify chronic lymphoproliferative disease, and for quantification of CD34-positive stem cells by high-dose chemotherapy with autologous stem cell support. Flow cytometry is routinely used for classifying non-malignant haematological disease like congenital and acquired immunodeficiencies. In diagnostic pathology, DNA flow cytometry is used for analysis of DNA ploidy and S-phase fractions in malignant tumours. DNA aneuploidy is a strong prognostic factor in some childhood tumours and in gynaecological malignant tumours, and flow cytometry is used in tumour biology for measurements of growth and apoptotic cell death. Furthermore, the method is an important research tool in many areas of biomedical research for detection of structural and functional cellular features.