Infants of <30 wk gestation often require respiratory support for several weeks and may develop bronchopulmonary dysplasia (BPD), which is associated with long-term pulmonary disability or death in severe cases. To examine the status of surfactant in infants at high risk for BPD, this prospective study analyzed 247 tracheal aspirate samples from 68 infants of 23-30 wk gestation who remained intubated for 7-84 d. Seventy-five percent of the infants had one or more surfactant samples with abnormal function (minimum surface tension 5.1-21.7 mN/m by pulsating bubble surfactometer), which were temporally associated with episodes of infection (p = 0.01) and respiratory deterioration (p = 0.005). Comparing normal and abnormal surfactant samples, there were no differences in amount of surfactant phospholipid, normalized to total protein that was recovered from tracheal aspirate, or in relative content of phosphatidylcholine and phosphatidylglycerol. Contents of surfactant proteins (SP) A, B, and C, measured in the surfactant pellet by immunoassay, were reduced by 50%, 80%, and 72%, respectively, in samples with abnormal surface tension (p < or = 0.001). On multivariable analysis of all samples, SP-B content (r = -0.58, p < 0.0001) and SP-C content (r = -0.32, p < 0.001) were correlated with surfactant function. We conclude that most premature infants requiring continued respiratory support after 7 d of age experience transient episodes of dysfunctional surfactant that are associated with a deficiency of SP-B and SP-C.