To identify possible sources of cytomegalovirus infection in pregnant women, we studied seven families with a recent case of congenital or maternal cytomegalovirus infection and a history of maternal contact with a young child shedding the virus. We used restriction-endonuclease techniques to compare the DNA of viral isolates collected from family members. Five families contained an infant who had congenital or perinatal infection, a mother who had had evidence of primary infection during her most recent pregnancy, and a child less than three years of age who was excreting cytomegalovirus. All five of the young children attended day-care centers at least part-time. In each of these five families, strains from family members were identical, and it is most likely that the toddler-aged child was the source of the virus for both the mother and the fetus or infant. In two other families, acquisition of cytomegalovirus by children in a day-care center was followed by seroconversion in the mother along with excretion of a strain of the virus identical to that in her child, as demonstrated by restriction-endonuclease analysis. Five of the seven fathers were tested for antibody to cytomegalovirus; four were seronegative, ruling them out as a source of infection in the mothers. These results not only strengthen evidence for the transmission of cytomegalovirus from child to mother but also indicate that infections acquired by a mother from a child can be transmitted to her fetus.