Cytomegalovirus (CMV) infections are widespread and usually asymptomatic; however, the virus may persist as a latent or chronic infection. The relatively frequent incidence and the severity of the disease in newborns and immunosuppressed individuals clearly establishes this agent as an important human pathogen. CMV infection can be classified as congenital (acquired before birth), perinatal (acquired at birth) and postnatal (acquired after birth). The prognosis for congenitally infected infants who are asymptomatic at birth must be guarded. Antibodies of the IgM class are produced during the first 2-3 weeks of infection with CMV and exist only transiently in most patients. Serologic procedures which measure the presence of IgM anti-CMV and IgG anti-CMV help to discriminate between primary and recurrent infections since IgM anti-CMV are rarely found in recurrent infections.