The cytomegaloviruses (CMV), members of the herpesvirus group. are widely disseminated; however, among healthy. nonpregnant adults these viruses are not usually responsible for serious illness or disability. However. CMV infections associated with pregnancy are an important cause of prenatal infection and are significantly associated with sensorineural injury and mental retardation in the offspring of infected mothers. CMV infections also constitute a significant threat to patients with disseminated malignancies. to allograft recipients. and to others who are debilitated and immunosuppressed. The development of a program of immunization against CMV has been proposed as a means to control prenatal infection and injury as well as to reduce the relevant morbidity and mortality of these infections among allograft recipients and others at high risk. In this review the potential benefits. problems. and risks associated with CMV immunoprophylaxis are discussed and results of preliminary trials of prototype CMV vaccines are presented and evaluated.