In a prospective Swedish study started in 1977 and still in progress 10 328 newborn infants in an urban district were investigated for cytomegalovirus (CMV) excretion in the urine by the virus isolation test. Congenital infection was found in 50 cases (0.5%). Of 47 infected infants with known clinical status at birth 9 (19%) had hepatomegaly, splenomegaly, jaundice and/or petechiae. The symptoms were moderate or mild. Of the infants followed up, 2 (25%) of 8 neonatally symptomatic ones and 3 (9%) of 35 asymptomatic ones developed neurologic sequelae. Altogether 5 (12%) of 43 had permanent neurologic symptoms corresponding to 0.06% in the general population. The children ranged in age from 6 months to 4 yr at the last examination. 21 mothers of the 47 infants with known status at birth had a confirmed or presumed primary infection, 15 a confirmed or presumed secondary infection and 11 an undetermined type of infection. Of the 5 infants with neurologic sequelae, 1 with a grave psychomotor retardation and deafness was born to a mother with a primary infection in the 1st trimester; 1 infant with a moderate retardation and 3 deaf infants were all exposed to confirmed or presumed secondary maternal infections. Prospective serological studies of maternal sera in early pregnancy would have suspected only the gravely retarded infant to be at risk.