Subjects were randomized to either standard mechanical ventilation or high frequency ventilation. The principal endpoint was the incidence of bronchopulmonary dysplasia defined as: the need for supplemental oxygen on the 28th postnatal day and for more than 21 of the first 28 days after birth; and abnormal chest radiographic findings that persisted until the 28th day of age. Other endpoints included the need for ventilatory support, the incidence of crossover from one form of ventilatory support to the other, and mortality rate before the 28th day of postnatal age. Adverse effects considered were pulmonary air leaks, severe intracranial hemorrhage, and periventricular leukomalacia.