Aims: To estimate the cost-effectiveness of isavuconazole compared with the standard of care, voriconazole, for the treatment of patients with invasive fungal infection disease when differential diagnosis of the causative pathogen has not yet been achieved at treatment initiation.Materials and methods: The economic model was developed from the perspective of the UK National Health Service (NHS) and used a decision-tree approach to reflect real-world treatment of patients with invasive fungal infection (IFI) prior to differential pathogen diagnosis. It was assumed that 7.8% of patients with IFI prior to differential pathogen diagnosis at treatment initiation actually had mucormycosis, and confirmation of pathogen identification was achieved for 50% of all patients during treatment. To extrapolate to a lifetime horizon, the model considered expected survival based on the patients' underlying condition. The model estimated the incremental costs (costs of drugs, laboratory analysis, hospitalization, and management of adverse events) and clinical outcomes (life-years (LYs) and quality-adjusted life-years (QALYs)) of first-line treatment with isavuconazole compared with voriconazole. The robustness of the results was assessed by conducting deterministic and probabilistic sensitivity analyses.Results: Isavuconazole delivered 0.48 more LYs and 0.39 more QALYs per patient at an incremental cost of £3,228, compared with voriconazole in the treatment of patients with IFI prior to differential pathogen diagnosis. This equates to an incremental cost-effectiveness ratio (ICER) of £8,242 per additional QALY gained and £6,759 per LY gained. These results were driven by a lack of efficacy of voriconazole in mucormycosis. Results were most sensitive to the mortality of IA patients and treatment durations.Conclusions: At a willingness to pay (WTP) threshold of £30,000 per additional QALY, the use of isavuconazole for the treatment of patients with IFI prior to differential pathogen diagnosis in the UK can be considered a cost-effective allocation of healthcare resources compared with voriconazole.