OBJECTIVES: In breast cancer, primary prophylaxis with pegfilgrastim has been shown to improve health outcomes but its cost-effectiveness has not been evaluated in the French setting. Filgrastim is often used for less than the recommended 11 days (e.g., 5-6 days), which has been associated with sub-optimal outcomes. This study compared the cost-effectiveness of pegfilgrastim versus 11- and 6-day filgrastim primary prophylaxis in women with stage I-III breast cancer receiving chemotherapy with moderate to high FN risk in France. METHODS: A decision-analytic model was constructed from a health care payer's perspective. Costs included drugs, drug administration, FN-related hospitalizations and subsequent costs, and were based on ex-factory price listing and DRG Tariff. Effectiveness was measured as FN avoided and life-year-gained (LYG). FN risk (varied by days of filgrastim), FN case-fatality, relative dose intensity (RDI), and the impact of RDI on survival were based on a comprehensive literature review and expert panel validation. Breast cancer mortality and all-cause mortality were taken from official statistics. Model robustness was tested using sensitivity analyses. RESULTS: Compared with 11 days of filgrastim, pegfilgrastim saved costs and was more effective. Compared with 6-day filgrastim, pegfilgrastim avoided 10.5 absolute percentage point of FN (17.5% vs. 7%). The incremental cost-effectiveness ratio (ICER) was €10,295 per FN avoided. The average life expectancy was 16.27 years with pegfilgrastim and 16.16 years with filgrastim, yielding an ICER of €9652/LYG. Age of diagnosis and cancer stage had minimal impact on the results. Key influencing factors included relative costs of drugs and relative risk of FN. CONCLUSIONS: Use of pegfilgrastim in France may dominate 11-day use of filgrastim and is cost-effective compared to 6-day use of filgrastim. The cost-effectiveness ratio is significantly below the commonly used threshold for cost-effectiveness ratios in Europe.