Subclassification of leukemias in childhood by cytochemical methods or blast size is arbitrary to some extent. The increased knowledge of physiological development of hematopoetic cells allows to classify these diseases according to the degree of differentiation of the cells involved. Immunological cell-membrane structures are used as for markers differentiation. In this way, 4 types of lymphoblastic leukemias and 5 types of myeloid leukemias can be diagnosed. This classification can help to answer clinical and theoretically important questions. In ALL and during the blast crisis of CML, new groups with increased risk are defined, important for the choise of initial therapy, and evaluation of therapeutic trials. The immunological markers can help to detect already small numbers of blasts at the beginning of a hematological relapse. Transformations of the blast type during the course of the disease can be explained. Experiments in animals indicate that an immunological classification of leukemias correlates with differences in pathogenesis and etiology.