In a comparison of the outcome of high-frequency oscillatory ventilation (HFO) and conventional mechanical ventilation (intermittent mandatory ventilation (IMV] in newborn infants, the degree of late pulmonary damage in these infants was assessed in a multicenter trial by examining their pulmonary status, including pulmonary function test results at 9 months of corrected age. A total of 432 infants were followed, 222 in the IMV group and 210 in the HFO group. Two-hundred twenty-three infants had their pulmonary mechanics measured, 118 in the IMV group and 105 in the HFO group. There were no significant differences between the two groups in conditions known to predispose infants to chronic lung disease. At 9 months of age, both groups has similar growth and a similar incidence of respiratory tract infections and hospital readmissions, and of retractions and episodes of wheezing. None of the pulmonary mechanics measurements differed. Forced expiratory flow at functional residual capacity was decreased (132 +/- 86 vs 135 +/- 92 ml/sec in the IMV and HFO groups, respectively), peak-to-peak esophageal pressure change was elevated (14.4 +/- 5.7 vs 13.5 +/- 5.7 cm H2O), dynamic compliance was in the low normal range (1.2 +/- 0.5 vs 1.3 +/- 0.6 ml/cm H2O/kg), and total pulmonary resistance was elevated (63 +/- 43 vs 57 +/- 34 cm H2O/L/sec) when the measurements were compared with normal values. The results indicate that in both groups, 30% to 40% of infants survived with chronic pulmonary changes similar to those described in infants with bronchopulmonary dysplasia. The use of high-frequency ventilation, in comparison with IMV, did not improve long-term pulmonary outcome.