To determine whether early use of high-frequency jet ventilation reduces neonatal mortality or pulmonary morbidity rates, we randomly selected 42 infants with clinical and radiographic evidence of severe respiratory distress syndrome to receive either high-frequency jet ventilation or conventional ventilation. Separate sequential analyses (two-sided, α=0.05, power=0.95 to detect 85:15 advantage) were performed for mortality rates, air leaks, bronchopulmonary dysplasia, intraventricular hemorrhage, and assignment crossover, and a combined analysis was performed, with death overriding other outcome variables. Enroliment was completed when the combined analysis reached the sequential design boundary indicating no treatment difference. Mortality rates (19% among infants recelving high-frequency jet ventilation vs 24% among infants receiving conventional ventilation), the incidence of air leaks (48% vs 52%), bronchopulmonary dysplasia (39% vs 41%), and intraventricular hemorrhage (33% vs 43%), and assignment crossovers (14% vs 24%) did not differ significantly between the treatment groups. We conclude that early use of high-frequency jet ventilation does not prevent or substantially reduce mortallity or morbidity rates associated with assisted ventilation. © 1990 Mosby-Year Book, Inc.