Resistance to antiviral drugs can be a severe problem in transplant recipients. Mutations in the HCMV phosphotransferase-gene (UL97) and the polymerase-gene (UL54) are responsible for resistance against ganciclovir (GCV), cidofovir (CDV) and foscarnet (PFA). Most frequently mutations in the UL97-gene are associated with resistance to GCV. Resistance against PFA and CDV is associated to mutations in the UL54-gene. There are only few reports about multidrug-resistance with mutations in both genes in patients after allogeneic haematopoietic cell transplantation (HCT).