Neutropenia is a common and potentially dangerous adverse effect of cancer chemotherapy. It also often necessitates a reduction or delay in dose, thus compromising efficacy. The human granulocyte colony-stimulating factor filgrastim has been proven to have a good safety profile and to be effective in accelerating neutrophil recovery and reducing the incidence of infections following myelosuppressive chemotherapy. However, its short serum half-life necessitates daily administration. Pegylated filgrastim (pegfilgrastim) was developed by attaching a polyethylene glycol moiety to the filgrastim protein. Pegfilgrastim binds to the same cell surface receptor on neutrophils and their precursors as filgrastim and stimulates the proliferation and differentiation of neutrophils through the same mechanism. However, because of the pegylation, it is minimally cleared by the kidneys and has a much longer serum half-life. Furthermore, its clearance is neutrophil dependent and thus 'self-regulated'. Pegfilgrastim is administered as a single subcutaneous injection per cycle of chemotherapy. In clinical trials it has been shown to be comparable in efficacy to filgrastim in decreasing the incidence of infection as manifested by febrile neutropenia following chemotherapy. Its safety profile and tolerability are similar to those of filgrastim.