Abstract Two transplant recipients (1 kidney and 1 hematopoietic stem cell) received maribavir (MBV) after cytomegalovirus (CMV) infection clinically resistant to standard therapy. Both patients achieved CMV DNA clearance within 30 and 18 days; however, the UL97 C480F variant emerged, causing recurrent CMV infection after a cumulative 2 months of MBV and 15 or 4 weeks of ganciclovir treatment, respectively. C480F was not detected under ganciclovir before MBV treatment. Recombinant phenotyping showed that C480F conferred the highest level of MBV resistance and ganciclovir cross-resistance, with impaired viral growth. Clinical follow-up and genotypic and phenotypic studies are essential for the assessment and optimization of patients with suspected MBV resistance. A cytomegalovirus UL97 C480F mutation emerged after maribavir treatment in 2 transplant recipients (1 kidney, 1 hematopoietic stem cell). Phenotypic assay confirmed that C480F confers a high level of maribavir resistance and ganciclovir cross-resistance, with a decreased replicative capacity.