The occurrence of Ureaplasma urealyticum and Mycoplasma hominis in the airways and the association of these microorganisms with chronic lung disease was studied in preterm infants with a gestational age less than 30 weeks. Tracheal aspirates from 49 preterm infants were cultured; 14 (29%) infants were positive for U. urealyticum, and 1 (2%) was positive for M. hominis. Of the 16 patients who developed lung disease, 6 (38%) were positive for U. urealyticum, while the expected number of Ureaplasma-positive patients in this group, based on the overall incidence of Ureaplasma, was 4.6. On the other hand, 8 patients were positive for U. urealyticum but did not develop chronic lung disease, nor did samples taken from 10 patients with chronic lung disease show any Ureaplasma growth. From these data we conclude that colonization of the airways with U. urealyticum has no significant role in the development of chronic lung disease in preterm infants in the Finnish (Caucasian) population.