We studied Ureaplasma urealyticum colonization in 93 intubated infants (gestational ages 23-40 weeks) in our neonatal intensive care unit by obtaining cultures from endotracheal aspirate and nasopharynx during their first week of life. Eighteen infants had positive cultures, giving a colonization rate of 19%. No infant more than 30 weeks' gestation had a positive culture. The infants with positive cultures had a significantly lower gestational age and birth weight (p < 0.009 and p < 0.005), with a colonization rate of 33% in infants less than 1000 g. Among the infants with positive cultures, 10 of 17 developed chronic lung disease in contrast with 21 of 72 infants with negative cultures. The development of chronic lung disease and duration of oxygen requirement was strongly associated with immaturity but only weakly with Ureaplasma urealyticum.