Patients are randomized to receive subcutaneous G-CSF at one of two different doses or no G-CSF (observation) for 24 weeks. Patients who experience ANC < 500 cells/mm3 on two consecutive occasions at least 24 hours apart prior to completing the 24-week study period will be considered to have reached the primary study endpoint; those in the observation group who reach the primary endpoint prior to week 24 may begin receiving G-CSF for the remainder of the study period. After 24 weeks, patients may continue G-CSF on a compassionate basis at the investigator's discretion.