Once considered to be an infrequent cause of human disease, human cytomegalovirus (CMV) is now recognized as being one of the most common congenital infections leading to birth defects, and furthermore, it is a common opportunistic infection associated with serious illness and death in immunocompromised patients. CMV has been assigned to the herpes family of viruses. It is an enveloped DNA virus. About 50-60 percent of Danish blood donors have anti-CMV. This figure is dependent on age, race, socioeconomic status, sexual behaviour and geographical area. Patients exhibiting symptoms and signs of primary or reactivated CMV infection should be further evaluated with appropriate laboratory tests, as treatment of CMV disease with antiviral chemotherapy such as ganciclovir or foscarnet, has proven to be effective.