Cytomegalovirus (CMV) pneumonia has been one of the most important opportunistic infections in the immunocompromised host. The disease is particularly severe in allogeneic bone marrow transplant recipients, with an almost 100% mortality rate in untreated patients. During the last few years, major advances in diagnostic techniques that allow more rapid detection of CMV in bronchoalveolar lavage have resulted in earlier diagnosis and thereby earlier commencement of therapy. Furthermore, the diagnosis of CMV infection in the blood using antigenemia assay or polymerase chain reaction has allowed the development of preemptive therapy strategies, substantially decreasing the risk for the development of CMV pneumonia. Ganciclovir or foscarnet combined with intravenous immunoglobulin has improved the outcome in some patients with established disease. However, resistance of CMV to antiviral agents is becoming an increasingly important problem, particularly in patients with the acquired immunodeficiency syndrome.