Infants with congenitally (38) and natally (17) acquired cytomegalovirus infection were prospectively studied by means of virologic and multiple serologic assays. These infections were characterized by chronic viral excretion (measured in years). The quantity of virus excreted in the urine during early infancy was significantly greater in infants who acquired infection in utero, particularly amon those born with overt disease; thereafter, all three groups (congenital symptomatic, congenital asymptomatic, and natal) excreted similar amounts of virus. The patterns of antibody responses, particularly the fluorescent antibody response to the early antigen and the complement-fixing antibody response, further indicated that congenitally infected infants (especially symptomatic ones) bear a greater antigenic burden than do natally infected infants. From a diagnostic standpoint, the test for fluorescent antibody to the late antigen was the most sensitive assay, whereas the test for complement-fixing antibody proved to be the least useful, The indirect hemagglutination assay, although performed only in infants with natal infection, was only slightly less sensitive than the fluorescent antibody procedure; by the former technique, diagnostic rises were detected in all but one infant after the onset of viruria.