Pegfilgrastim is a covalent conjugant of filgrastim (a recombinant human granulocyte colony-stimulating factor) and monomethoxypolyethylene glycol. It is administered as a single dose per myelosuppressive chemotherapy cycle to decrease the incidence of infection, as manifest by febrile neutropenia, in patients with nonmyeloid cancer. Pegfilgrastim increases the terminal elimination half-life and decreases the apparent serum clearance of the drug in patients with nonmyeloid cancer. Serum concentrations of pegfilgrastim remain elevated during neutropenia but decline when the neutrophil count increases. In phase III trials in patients with breast cancer and in a phase II trial in patients with non-Hodgkin's lymphoma or Hodgkin's disease, the mean duration of grade 4 neutropenia and the time to absolute neutrophil recovery during cycle 1 of chemotherapy was similar in recipients of single-dose pegfilgrastim or daily filgrastim. In the larger of two phase III trials in patients with breast cancer, the incidence of febrile neutropenia over four cycles of chemotherapy was significantly lower in recipients of single-dose pegfilgrastim than that in recipients of daily injections of filgrastim. Moreover, the mean duration of grade 4 neutropenia in cycles 2 to 4 of chemotherapy was significantly lower in recipients of pegfilgrastim than that in recipients of daily filgrastim. In comparative trials, there were no differences in the incidence and severity of adverse events, including skeletal pain, between single-dose pegfilgrastim and daily filgrastim in patients with nonmyeloid cancer receiving myelosuppressive chemotherapy.