G-CSF is the major growth factor involved in the production of neutrophilic granulocytes. G-CSF exerts its function via the activation of a membrane receptor that belongs to the super-family of hematopoietin receptors, also referred to as class I cytokine receptors. In this review we summarize the current knowledge of the function of the G-CSF receptor in normal granulopoiesis, as well as in some patients with severe congenital neutropenia and acute myeloblastic leukemia, diseases characterized by disturbed myeloid maturation.