This article presents 3 cases of immunocompromised patients for whom therapeutic drug monitoring of ganciclovir in combination with cytomegalovirus viral load measurement was used to guide treatment. The first patient is diagnosed with thymoma A, the second is a heart transplant recipient, and the third is an HIV-positive patient. These patients were all diagnosed with cytomegalovirus and treated with ganciclovir. Our case studies illustrate how therapeutic drug monitoring-guided dosing can be helpful in the management of these complex cases.