In a prospective study 225 (35%) of 640 pregnant women who delivered at term had vaginal colonization with Ureaplasma urealyticum at the time of delivery. One hundred ninety-three full term infants born to U. urealyticum-colonized mother were cultured from the throat, eyes and vagina within the first 3 days of life. One hundred seven infants (55%) had at least one culture site positive for U. urealyticum (throat 41%, eyes 20%, vagina 40%). Rupture of membranes for greater than or equal to 12 hours and the mode of delivery did not affect vertical transmission of U. urealyticum. We were able to follow 108 infants during the first 3 months of life. Sixty-eight, 33 and 37% of the infants who were initially colonized with U. urealyticum in the throat, eyes and vagina, respectively, were still colonized when the follow-up cultures were obtained 3 months later. Fourteen of the 108 infants whom we followed developed a lower respiratory tract illness. In the pharyngeally colonized infants there was no increased risk for lower respiratory tract illness during early infancy compared with the pharyngeally noncolonized infants.