Infection rates with cytomegalovirus among children in three day care centers were compared to that found in a group of children cared for in the home who were from a similar socioeconomic background. Rates of viral excretion for Day Care Centers 1, 2 and 3 were 41% (28 of 68), 26% (15 of 58) and 55% (34 of 62), respectively, with a combined rate of infection for all children in day care of 41% (77 of 188). In contrast 15% (10 of 66) of children in home care were seropositive to cytomegalovirus, and only 2 of 25 (8%) were shedding virus (P less than 0.001). Although the median age of the group in home care was slightly lower, the two groups of children were similar in sex, race, breast-feeding and parental ages and educational background. The only other notable difference between the children in day care and those in home care was the environment in which they received care for almost 40 hours every week. Rates of infection varied also among the age groups within each center, with the highest rate within each center occurring in children 25 to 36 months old. Day care for young children is likely to be associated with high rates of infection with cytomegalovirus. The mechanisms of transmission that are responsible for these higher than expected rates of infection remain to be defined.