Five pathologic findings have been found to be associated with ventilatory-induced lung injury in the premature baboon model of hyaline membrane disease. They are: (1) tracheal and major bronchial lesions, (2) small airway changes, (3) inflation pattern aberrations, (4) bronchoalveolar hemorrhage, and (5) air leak problems. The use of immediate high frequency oscillatory ventilation (HFOV) prevents the bronchiolar overdistension (small airway), atelectasis (inflation pattern), and air leak problems. The lesions in the trachea and large bronchi of prematures all show injury secondary to prolonged intubation, but after HFOV the lesions are no worse than those seen in PPV-treated tracheas. HFOV does increase the incidence of bronchoalveolar hemorrhages, and this lesion plus some of the non-pulmonary complications will require further investigation.