We screened parents of children from three previously studied day-care centers where children have maintained high rates of cytomegalovirus (CMV) excretion, as well as parents of children not in day-care centers (controls), for antibody to CMV. Longitudinal serologic follow-up of seronegative parents revealed that 14 of 67 with children in the day-care centers acquired CMV, as compared with none of 31 controls (P less than 0.003). All 14 parents who seroconverted had a child who was shedding CMV in saliva or urine. Among the day-care group, acquisition of CMV occurred in 14 of 46 parents of children who shed CMV, as compared with none of 21 whose children did not excrete the virus (P less than 0.0001). There was no correlation between parental seroconversion and sex, race, age, number of years of education, marital status, occupation, or number of children in the home. CMV infection occurred in 9 of 20 parents (45 percent) with a child shedding CMV who was 18 months of age or less at enrollment. We conclude that children often transmit CMV to parents and could be an important source of maternal CMV infection during pregnancy.