The occurrence and possible consequences of cytomegalovirus (CMV) infections were studied in 200 mothers and their children by means of immunofluorescent antibody assays in serum, virus isolation from urine and regular clinical and neurological examinations. The prospective study covered the time from early pregnancy to 1 year post partum. The frequency of intrauterine infections was 2%, while 30% of the children became perinatally infected as indicated by the onset of virus excretion and an antibody response at the age of 2--4 months. Later on the occurrence of CMV infections declined sharply. 23 mothers had no CMV antibodies and none of their children contracted CMV during the first year of life. Maternal antibodies seemed unable to protect the child from CMV infections or to delay the onset of virus excretion in perinatally infected children. Intrauterine infections did not correlate with significant increases in the antibodiy titres of the mothers or the presence of IgM antibodies either in the mother's sera or in the cord sera. Perinatal infections were often associated with the presence of IgM antibodies both in the child and in the mother and in these mothers significant increases in CMV antibody titres were frequently seen, probably indicating an activated latent infection. Immunofluorescent antibody assay correlated well with virus isolations and was more sensitive than complement-fixing antibody assay.