Two hundred and fifty three infants were screened for cytomegalovirus (CMV) in the urine at birth and were followed up at regular intervals for one year. Twelve per cent (of 249) were excreting virus at 3 months, and 20% (of 234) at 12 months. In all cases infection was subclinical. The major factors determining risk of acquiring infection were the mother's serological state and whether the infant was breast fed. There was no association with social class, mother's age, or whether the child had been in a special care baby unit or a postnatal ward. By one year 33% (of 123) of infants of seropositive mothers had acquired CMV infection compared with 4% (of 123) born to seronegative mothers. Twenty per cent (17) of seropositive women who breast fed had virus isolated from their breast milk on at least one occasion, and 76% (13) of their infants became infected. In four mother-infant pairs comparison of CMV isolates from the mother's milk and the child's urine was made by restriction endonuclease digestion; in each pair infection had apparently occurred with the same strain of virus. All 13 infected infants followed up for three years were still shedding virus. Infection with CMV is common in infancy, and virus shedding persists for years. Congenital infection cannot be distinguished from acquired infection unless the presence of CMV in the urine is identified within three or four weeks after birth, even when clinical problems suggestive of congenital infection are present.