Flow cytometric quantitation in chronic leukemias provide relevant information in different areas. It allows the characterization of additional leukemia associated parameters, like over- or under- expression as compared to normal counterparts, that is suitable for the diagnosis of malignancy or for residual disease evaluation. It improves scoring systems for the differential diagnosis between the different chronic malignancies. It allows to find original pronostic parameters and improves the comparison of different series due to a better definition of positivity. It helps to the monitoring of immune based therapeutic regimens. It provides new relevant pathophysiological informations, for instance in the study of apoptosis, as a tool for quantitative evaluation of the apoptotic process itself and for the study of its mechanism. Since its first description by Minot and Isaacs in 1924, chronic lymphocytic leukemia (CLL) has been extensively studied, due in part as it has been demonstrated as the more frequent leukemia in the Western world (Gale and Foon 1987). In addition, clinical features, cell morphology, immunological markers, histopathology and molecular genetics have been used progressively to define and characterize a wide spectrum of chronic lymphoproliferative disorders which had been previously gathered in the same entity than CLL (Benett et al., 1989, Catovsky and Matutes 1991, Harris et al., 194). Immunologic techniques are now routinely used for diagnosis in this context. Since there is no marker specific for each of these entities, scoring systems have been proposed to help diagnosis (Matutes and Catovsky 1994), which are currently investigated in multicentric studies. Flow cytometry is now widely used for immunophenotyping purposes. It allows, in addition to the determination of the percentage of positive cells, to determine the intensity of fluorescent staining, that can be converted into antigen density provided that reagents are used under saturating concentrations and correct standards of fluorescence are tested in parallel. The concept of antigen density evaluation appears to improve the efficiency of immune techniques in the monitoring of hemopoietic malignancies (Lavabre-Bertrand et al., 1994c). The present review will focus on the specific interest of immune quantitation in the study of chronic lymphoid malignancies, with a successive emphasis on diagnosis of malignancy, distinction between the different chronic lymphopathies, prognostic and therapeutic relevance, then on pathophysiological interest.