Cytomegalovirus (CMV) is a common virus that asymptomatically infects the majority of the population by early adulthood. After initial infection, the virus remains in a latent state in the host and can be reactivated under a variety of conditions. In contrast to the normal host, both primary and reactivated symptomatic CMV infections are common in certain immunocompromised patients including organ transplant recipients and patients with the acquired immunodeficiency syndrome (AIDS). One of the more common manifestations of the virus in this setting is a severe and often fatal interstitial pneumonitis. In transplant patients, this predictably occurs in the second or third month after transplantation and is characterized by severe hypoxemia and bilateral, symmetric interstitial infiltrates on chest x-ray. While the most direct and specific means of diagnosing CMV pneumonia is isolation of the virus from lung tissue homogenates cultured on human fibroblasts, techniques have recently been introduced using specific monoclonal antibodies or CMV DNA probes to rapidly identify the virus in tissue biopsy, cytologic specimens, or in tissue culture. These rapid techniques will become particularly important as antiviral agents with activity against CMV become available.