This study examined the association between Ureaplasma urealyticum colonization and the development of chronic lung disease (CLD) in 93 premature infants who were treated with surfactant and who had birth weights < 1251 g. Nasopharyngeal and tracheal cultures for U. urealyticum were obtained at 2 +/- 1 and at 14 +/- 1 days after birth and were positive in 17 (18%) of 93 patients. Infants born vaginally were 4.5 times more likely to be colonized than were those born by cesarean section. Colonization with U. urealyticum was associated with 1.66 (95% confidence interval, 1.24-2.20, P = .024) times the risk of developing CLD and with a greater incidence of > or = 2+ polymorphonuclear leukocytes in the tracheal aspirate at 2 +/- 1 days of age compared with uncolonized infants (P = .025). We conclude that U. urealyticum colonization is associated with CLD even after surfactant treatment. The presence of U. urealyticum is also associated with inflammatory cells in the tracheal aspirate.