The sequential pathological changes of the respiratory distress syndrome have been assessed in 200 lung biopsies and autopsy specimens over the past ten years. The initial microscopic changes consisted of thromboemboli in pulmonary arterioles. These were followed by congestion and interstitial edema and hemorrhage over the next 24 hours. Intra-alveolar hemorrhage next became evident and reached a peak at 72 hours. Following this, hyaline membranes became the predominant lesion. Bronchopneumonia may be superimposed on these changes at any point from this time on. If the patient survived for a matter of weeks or months prior to death, pulmonary fibrosis was the terminal lesion. The variable combination of these pathological changes explains the clinical manifestations of the respiratory distress syndrome.