Study Objectives. To study the impact of filgrastim 5 μg/kg given once/day through absolute neutrophil count (ANC) recovery on the duration of grade 4 neutropenia (ANC < 0.5 × 103/mm3) and time to ANC recovery. Additional objectives were to study the average number of filgrastim injections/cycle required to achieve ANC recovery and differences in outcome by cycle. Design. Combined analysis of two double-blind, randomized, multicenter trials. Patients. Two hundred twenty-two patients treated for breast cancer. Measurements and Main Results. All patients but one were evaluable for efficacy end points. Mean ± SD duration of grade 4 neutropenia was 1.7 ± 1.3 days in cycle 1; the duration decreased in cycles 2-4 to between 1.0 and 1.2 days. Fifty percent of patients had ANC recovery to 10 ×0 103/mm3 or greater by day 11 of the cycle, and 90% by day 13, corresponding to 10 and 12 days of filgrastim administration, respectively. Across all cycles, the mean ± SD number of filgrastim injections/cycle was 10.51 ± 1.70, with little variation among cycles. Conclusion. When filgrastim is administered as recommended, starting 24 hours after chemotherapy and continuing through an ANC of 10 × 103/mm3 or greater, neutrophil recovery is rapid and predictable. Because the first cycle of chemotherapy has the highest rates of neutropenia and febrile neutropenia, it seems prudent to administer growth factor support preemptively.