A common disorder of premature (or preterm) birth, defined by respiratory difficulty, which requires oxygen or assisted ventilation, and characteristic changes on chest xray. It is primarily the result of deficiency in surface-active lipids (contained within pulmonary surfactant) which serve to stabilize the air spaces (alveoli) within the lung. Although most cases of infant respiratory distress syndrome rarely result in a fatal outcome in developed countries, all require specialized care in a neonatal intensive care unit.