Cytomegalovirus (CMV), the leading cause of congenital viral infection, occurs commonly among children in group day care. Urinary or salivary excretion of CMV was found frequently among children in three centers serving mostly middle-income white families. Although there was center-to-center variation, CMV excretion was uncommon among infants under one year of age; peak rates of viral shedding, ranging from 44% to 100%, were noted for two-year-olds. A longitudinal study at a single center indicated that children usually acquired CMV during their second year of life and usually shed virus for two years or longer. The high prevalence of silent CMV infection among the children in day care argues against the exclusion of any child known to have CMV infection; such children have sometimes been excluded because of the potential risk of CMV transmission to pregnant workers.