An editorial notes that given the potent antiviral effect of letermovir, the issue of drug resistance will need to be investigated, especially in patients with breakthrough viraemia during the prophylactic period, and the incidence of postprophylaxis infection and late CMV disease must be assessed. Nevertheless, the potential availability of a new class of antiviral drug with high potency and a low side-effect profile, and one that could be used alongside existing therapy, is a welcome development. Letermovir was granted orphan drug status in the EU in June 2012.