A common disorder of premature (or preterm) birth, defined by respiratory difficulty, which requires oxygen or assisted ventilation, and characteristic changes on chest x-ray. 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.