Endotracheal aspirates from 200 infants who weighted less than or equal to 2500 g and who had evidence of respiratory disease were cultured within 24 h of birth for mycoplasmas, chlamydiae, viruses, and bacteria to evaluate the relation between lower respiratory tract infection and development of chronic lung disease and/or death. Ureaplasma urealyticum, an organism not visible on gram stain, not recovered on routine bacteriological media, and not susceptible to antibiotics commonly used to treat neonatal infections, was the single most common organism isolated. 14% of isolates were from infants born by caesarean section with intact membranes, which indicated that infection had occurred in utero. The findings probably represented true infection of the lower respiratory tract because the organism was recovered in pure culture in numbers greater than 10(3) from 85% of the infants, and also from the blood in 26% of infants. Those infants less than or equal to 1000 g with Ureaplasma urealyticum infection of the lower respiratory tract were twice more likely to have chronic lung disease or to die than were infants of similar birth-weight but who were uninfected, or infants greater than 1000 g. Very-low-birth-weight infants infected with ureaplasmas did not differ from those uninfected, either in demographic features or in potential risk factors for chronic lung disease.