The epidemiology of the cytomegalovirus and the characteristics of mother-to-foetus transmission indicate that most neonatal infections (0.5 to 2% of live births) are due to reactivation of a latent virus during pregnancy. These infections are, and probably remain, subclinical. Even in case of primary infection, there seems to be little risk for the foetus. However, the patent forms of the disease are so serious that an effective and safe immunization method is highly desirable.