Cytomegalovirus (CMV) infection is a common complication following solid organ transplantation that may severely affect the outcome of transplantation. Ganciclovir (GCV) and its prodrug valganciclovir are successfully used to prevent and treat CMV infection; however, in a small percentage of patients, CMV gene mutations may lead to drug resistance. GCV resistance is defined as increasing CMV viremia or progressive clinical disease during prolonged antiviral therapy, due to CMV gene mutation. This has emerged as a new challenge, especially because alternative drugs such as cidofovir and foscarnet have a number of important side effects. Here we report the case of a kidney transplanted patient who experienced life-threatening CMV disease, which initially appeared to be GCV-resistant, but was instead found to be associated with inadequate antiviral drug levels. The patient was then successfully treated by monitoring plasma GCV levels. We suggest using plasma GCV monitoring in the management of all cases of critical CMV disease, in which GCV resistance is suspected.