• We studied the natural history of cytomegalovirus (CMV) excretion among 79 children in a single day-care center over a 2½-year period. During the study interval, 28 children (35%) excreted CMV in their urine, or saliva, or both. The CMV acquisition rate among children who were initially culture negative was 12.6% per year. In such children, CMV excretion began 11 to 59 months after entry into day care. The duration of CMV excretion varied from 3.0 to 28.4 months, with a mean of 13.0 ±9.1 months for urine and 7.0±2.7 months for saliva. The quantity of CMV in saliva or urine was highest during the first three months of excretion, as high as 105 50% tissue culture infectious dose per milliliter. Children excreting CMV entered day care at a younger age (mean, 5.3±8.5 months for excretors vs 12.7±14.8 months for nonexcretors) and spent more hours in day care per week than the nonexcretors (mean, 41.8±9.0 h/wk for excretors vs 36.1±10.9 h/wk for nonexcretors).