The prevalence of cytomegalovirus (CMV) infection in 103 children attending a single day care center was compared with that of 62 children kept at home. Demographic features of the day care group were similar to those of the home care group; in both groups, most of the children were white, were from middle-income families with two or fewer children, and had parents with 15 or more years of education. Ten of 57 (18%) children in home care had serum antibody to cytomegalovirus (CMV) and 2/25 (8%) had viruria. In contrast, 59/103 (57%) of children from the day care center were shedding CMV in urine or saliva. Although less than 10% of infants aged less than 1 year were excreting CMV, 78% of infants studied between 12 and 18 months of age had CMV isolated from a saliva or urine specimen. Group day care appears likely to result in early acquisition of CMV. Excretion of CMV by children in day care centers could result in transmission of the virus to susceptible day care workers or mothers, with the potential risk of fetal infection.