Ureaplasma urealyticum is a common component of the vaginal flora during pregnancy. Although colonization of low birth weight infants with U. urealyticum occurs frequently, the actual rate of vertical transmission of U. urealyticum in preterm infants has not been determined. Sixty-five preterm infants (less than 37 weeks of gestation) born to mothers colonized with U. urealyticum had eye, throat, vagina and rectum cultured for U. urealyticum at 1, 3 and 7 days of age and weekly thereafter for the first month of life while the infants remained in the hospital. Thirty-eight infants (58%) had at least one culture site positive for U. urealyticum (eye, 8%; throat, 37%, vagina, 54%; and rectum, 18%). Vertical transmission was not affected by method of delivery or duration of rupture of amniotic membranes. The rate of vertical transmission of U. urealyticum was higher among infants with birth weight less than 1,000 g (89%) than among those with birth weight of 1,000 g or greater (54%) (P = 0.07). Chronic lung disease developed in 9 of the 65 (14%) infants; 8 were colonized with U. urealyticum. The high rate of ureaplasmal colonization and chronic lung disease in infants less than 1,000 g makes these infants a suitable target population for a clinical treatment trial to determine whether eradication of U. urealyticum would decrease the incidence of chronic lung disease.