BACKGROUND: Chronic rhinosinusitis (CRS) is one of the commonest chronic health problems among adults in the UK. Around 15% of CRS patients undergo functional endoscopic sinus surgery (FESS) annually after failing medical treatment. However, as incomplete resolution of symptoms or complications post-operatively is common, the post-operative management is considered to be as important as the surgery itself. A bioabsorbable corticosteroid-eluting sinus implant (CESI) (Propel®, mometasone furoate 370 µg) has been used as an alternative post-FESS treatment. OBJECTIVE: The objective of this study was to assess the cost effectiveness of the corticosteroid-eluting implant versus non-corticosteroid-eluting spacer following FESS for treatment of patients with CRS. METHODS: A decision tree model was developed to estimate the cost and effectiveness in each strategy. Costs and effects were estimated from a UK National Health Service (NHS) and personal social services perspective over a 6-month time horizon. Model pathways and parameters were informed by existing clinical guidelines and literature and sensitivity analyses were conducted to explore uncertainties in base-case assumptions. RESULTS: Over a 6-month time horizon, inserting CESI at the end of FESS is less costly (£4646 vs. £4655 per patient) and is the more effective intervention [total quality-adjusted life-years (QALYs) over 6 months 0.443 vs. 0.444] than non-corticosteroid-eluting spacers; hence, it is a dominant strategy. The probabilistic analysis results indicate that CESI following FESS has a 62% probability of being cost effective at the £20,000/per QALY willingness-to-pay threshold and 56% probability of being a cost-saving intervention. CONCLUSIONS: The use of CESI after FESS results in fewer post-operative complications than non-corticosteroid-eluting implants and may be a cost-saving technology over a 6-month time horizon. Although the cost of initial treatment with the CESI is greater, cost savings are made due to a reduction in the number of complications experienced.