OBJECTIVES: This study evaluated the cost-effectiveness of adding adjunctive granulocyte colony-stimulating factor (G-CSF) to antibiotics in the treatment of established febrile neutropenia (FN) among adult cancer patients undergoing chemotherapy. METHODS: A decision-tree model was created to compare two treatment options for established FN: (i) antibiotics alone without G-CSF (standard of care); and (ii) antibiotics plus adjunctive G-CSF (daily filgrastim until neutrophil recovery). The target population was a hypothetical cohort of adult cancer patients with solid tumours or lymphomas hospitalised with FN in Singapore. The analysis was performed from a hospital’s perspective over a 21-days time horizon. Costs and outcomes were not discounted. The main outcome measures included costs, quality-adjusted life year (QALY) and incremental cost-effectiveness ratio (ICER). One-way sensitivity analyses and probabilistic sensitivity analyses (PSA) were conducted to evaluate the robustness of the results. RESULTS: Compared with antibiotics alone, adjunctive therapy with G-CSF was a dominant strategy, incurring a cost-saving of US$125 per patient (G-CSF vs. Without G-CSF: US$9,110 vs. US$9,235) and additional health benefit of 0.0007 QALYs gained per patient (G-CSF vs. Without G-CSF: 0.0450 vs. 0.0443). Model results were robust against the parameter variations in the one-way sensitivity analyses, but increasing the cost of filgrastim beyond US$87/injection would increase the ICER to more than US$50,000/QALY. Furthermore, adjunctive therapy with G-CSF was the preferred strategy (dominant or cost-effective) in 83.7% of the model iterations at a willingness-to-pay threshold of US$50,000/QALY.  CONCLUSIONS: From a Singaporean hospital’s perspective, the adjunctive use of G-CSF is cost-effective in the treatment of FN among patients with solid tumours or lymphomas.