BACKGROUND: Prophylaxis with granulocyte colony-stimulating factor reduces the risk for febrile neutropenia (FN) in patients receiving myelosuppressive chemotherapy. OBJECTIVE: We estimated the incremental cost-effectiveness of primary prophylaxis (starting in cycle 1 of chemotherapy) with pegfilgrastim versus filgrastim in women with early-stage breast cancer receiving myelosuppressive chemotherapy in the United States. METHODS: A decision-analytic model was constructed from a health payer's perspective with a lifetime study horizon. The model considered direct medical costs and outcomes related to reduced FN and potential survival benefits due to reduced FN-related mortality and on-time receipt of full-dose chemotherapy. Sensitivity analyses were conducted. RESULTS: Pegfilgrastim was cost-saving and more effective (ie, dominant strategy) than 11-day filgrastim. The incremental cost-effectiveness ratio (ICER) for pegfilgrastim versus 6-day filgrastim was $12,904 per FN episode avoided. Adding the survival benefit due to reduced FN mortality and receipt of optimal chemotherapy dose yielded an ICER of $31,511 per quality-adjusted life year (QALY) gained and $14,415 per QALY gained, respectively. The most influential factors included inpatient FN case-fatality rate, cost of pegfilgrastim and filgrastim, baseline probability of FN, relative risk for FN between filgrastim and pegfil-grastim, and cost of administration of filgrastim. CONCLUSION: Pegfilgrastim was cost-saving compared with 11-day filgrastim and cost-effective compared with 6-day filgrastim from a health payer's perspective for the primary prophylaxis of FN in these women with early-stage breast cancer receiving myelosuppressive chemotherapy.