Voriconazole is used for the prophylaxis and treatment of invasive fungal infections (IFIs). The development of IFIs in patients being treated for haematological malignancies is associated with increased morbidity and mortality. As diagnosis of IFIs may be difficult, targeted prophylactic therapy is important in high-risk patients. Voriconazole is used in patients who have become resistant to itraconazole, and offers a broader spectrum of antifungal activity. Voriconazole is metabolised by CYP 2C19, which exhibits genetic polymorphism. Homozygous extensive metabolisers may have half the plasma concentration compared to homologous poor metabolisers. As a result in individuals prescribed the same dose of voriconazole, plasma voriconazole concentrations may vary up to 100-fold. Thus monitoring plasma concentration may help in dose optimisation and in monitoring adherence to medication. A pre-dose target plasma voriconazole concentration of 2-4 mg/L has been suggested for effective prophylaxis in patients being treated for haematological malignancies.