At the cellular level, expansion of haemopoiesis in chronic myeloid leukaemia (CML) must involve some imbalance in cell production along the myeloid maturation pathway. The relevant kinetic parameters are cell loss by apoptosis and differentiation and cell gain by proliferation (self-renewal). In spite of the predominance of the BCR-ABL-positive leukaemic cells, some BCR-ABL-negative, presumably normal, progenitor cells remain for long periods in chronic phase CML. Thus, understanding the kinetics of CML and normal progenitor cells may lead to therapeutic strategies capable of reducing malignant cell growth and reactivating normal haemopoiesis.