We studied cytomegaloviruses (CMV's) isolated from mothers and their children to determine whether recurrent infections and transmission to the fetus in immune women are due to reinfection or reactivaton of endogenous virus. Twenty-one strains of CMV from six mother-baby pairs, from two congenitally infected siblings, and from repeat isolates from four women were examined for genetic relatedness, by means of restriction endonuclease analysis of purified viral DNA. Although CMV's from unrelated persons were always different, strains from five to six congenitally infected babies were identical or very closely related to those from their mothers. Strains from congenitally infected siblings were also concordant, as were repeat isolates form three of four women. Major differences in viral DNA were detected in strains recovered from the same woman or from a mother-baby pair in only two cases. Endogenous CMV appears to be most frequent source of recurrent infection and intrauterine transmission in immune women; reinfection also occurs, but less commonly.