Recombinant human granulocyte colony-stimulating factor (rhG-CSF) is currently used for treatment of various types of neutropenia, treatment of aplastic anemia, mobilization of peripheral blood progenitor cells. However, rhG-CSF is not only a growth factor for the myeloid lineage, but it also acts as a modulator of neutrophil behavior. The aim of the present review article is to examine the following aspects of rhG-CSF therapy: 1) does rhG-CSF influence neutrophil functions, and in particular their microbicidal properties? 2) does rhG-CSF modify neutrophil phenotype? 3) If so, what are the mechanisms potentially involved? The author of the present article has been working in the field of rhG-CSF effects on neutrophil function, contributing original papers in peer-reviewed journals. In addition, the present review critically examines articles and abstracts published in journals covered by the Science Citation Index and Medline. Treatment with rhG-CSF causes enhancement of functions such as phagocytosis, superoxide anion generation, chemiluminescence, bacterial killing, and ADCC. Neutrophil phenotype changes after rhG-CSF administration: immediate effects cause direct activation of circulating neutrophils, but delayed effects are characterized by increased surface expression of important effector molecules directly involved in neutrophil functions, such as CD14, CD32, CD64. These effects may have useful clinical consequence in patients who show an increased risk of infections, such as cancer patients, subjects with hematologic diseases (myelodysplasia, aplastic anemia), congenital diseases characterized by neutropenia, and patients with AIDS. Other changes which characterize neutrophils after rhG-CSF administration are represented by significant impairment of CD16 expression, chemotaxis, and reduced in vivo migration of neutrophils to inflammatory sites. These effects may be explained by bone marrow modification due to rhG-CSF therapy. In fact, treatment with rhG-CSF causes a significant acceleration of transit time of cells belonging to the myeloid lineage, along with amplification of the mitotic pool and a relative decrease of elements of the post-mitotic pool. It is possible that, because of the accelerated bone marrow transit time of myeloid cells, rhG-CSF causes a relative immaturity of circulating neutrophils. It is known that both CD16 expression and chemotaxis properties are acquired by neutrophils in the late stages of maturation, but the time necessary to acquire full functional maturity seems to be shortened by rhG-CSF administration, and this kinetic aspect may play a non-negligible role in the modification of neutrophil behavior.