In a prospective study of 148 children from urbanized southern Finland 3 were found to be congenitally and 48 perinatally infected with cytomegalovirus (CMV), while 6 developed "late" infection during the first year of life. During pregnancy and the first year after delivery 23 of the mothers had no CMV antibodies; none of the children of these seronegative mothers developed any type of CMV infection. Fresh blood exchange transfusions did not increase the risk of CMV infection. The data support the hypothesis that the mother is the source of perinatal CMV infection. Children with a low birthweight not due to prematurity, and first children seem to run a greater risk of acquiring perinatal CMV infection. If the child is breast fed up to the age of 2 months the risk seems to be increased. Perinatal CMV infection gave rise to no symptoms or signs and had no effect on growth or on motor and psychosocial development during the first year of life.