Ureaplasma urealyticum is a common inhabitant of the urogenital tract of pregnant women. This mycoplasma is transmitted from a colonized woman to her newborn infant in one of three ways: (1) in utero either transplacentally from the mother's blood or by an ascending route secondary to colonization of the mother's urogenital tract; (2) at delivery by passage through a colonized birth canal; and (3) postnatally by horizontal or nosocomial transmission. The rate of vertical transmission ranges from 18% to 55% among full-term infants and from 29% to 55% among preterm infants. The rate of vertical transmission is not affected by method of delivery but is significantly increased when chorioamnionitis is present. Moreover, colonization of the infant with U. urealyticum occurs despite delivery by cesarean section with intact fetal membranes; ureaplasmal infection of the amniotic sac without rupture of membranes has been well described. Ultimately, the prevalence of infant colonization will depend on the prevalence of maternal colonization in a given population. However, the rate of colonization appears to be higher in very-low-birth-weight (VLBW) infants. Given this finding, as well as the potential for serious disease that could be attributed to U. urealyticum in this population, it seems reasonable to target the VLBW infant for a clinical treatment trial to determine if eradication of this organism will decrease the incidence of chronic lung disease.