No bronchopulmonary dysplasia was defined by requiring treatment with room air with oxygen saturation of more than 90% or passing a timed, continuously monitored oxygen-reduction test.[7] The physiologic definition of bronchopulmonary dysplasia reduced the overall rate of bronchopulmonary dysplasia and reduced the variation among centers. The physiologic definition may facilitate the measurement of bronchopulmonary dysplasia as an outcome in clinical trials and the comparison between and within centers over time.