Ureaplasma urealyticum is a common inhabitant of the urogenital tract of pregnant women. Although colonization of newborn infants with U. urealyticum has been documented previously, the actual rate of vertical transmission has not been determined. Cervical cultures for U. urealyticum were performed on 1315 pregnant women on admission to the labor suite. A positive culture was found in 810 (62%). Eye, nasopharyngeal and/or throat, vaginal and rectal cultures were obtained in the first 5 days of life from 132 full term infants born to mothers colonized with U. urealyticum. Fifty-nine infants (45%) had at least one culture site positive for U. urealyticum (eye, 4%; nasopharynx 24%; throat, 16%; vagina, 53%; and rectum, 9%). None of the infants had evidence of disease caused by U. urealyticum during the nursery stay. Vertical transmission was not affected by the method of delivery. However, among the vaginally delivered infants, rupture of membranes greater than 1 hour correlated with an increased rate of vertical transmission of U. urealyticum (52%) compared with rupture of membranes less than or equal to 1 hour (22%) (P less than 0.05). Because vertical transmission of U. urealyticum occurs frequently, caution must be exercised when attributing disease to U. urealyticum based solely on positive cultures of mucosal surfaces.