Acute chest syndrome (ACS) is characterized by chest pain with dyspnea and recent radiologic abnormalities, and is an acute lung complication whose problem is one of etiology. Alveolar hypoventilation linked to infarcts of the thoracic ribs, thoracoabdominal trauma, subdiaphragmatic pain, the administration of analgesics causing respiratory depression, or sleep disturbance, is a frequent cause of ACS. Bronchoalveolar lavage has revealed the frequency of fat embolism following infarcts in the long bones. Pulmonary vascular occlusion, due to thrombosis or emboli, is rare, as are the infectious pneumonia and pulmonary edema. The pathogenetic mechanisms consist of an alteration of the rheological properties of the blood, the existence of an hypercoagulability state, specific interactions between the abnormal sickle cells and the vascular endothelium, and a dysregulation of the vascular reactivity. Research centered around NO biology has led to an expanded understanding of the critical interdependence of NO, hemoglobin, and the microvasculature. An anemic patient with ACS suffers from loss of pulmonary scavenging and hypoxic pulmonary vasoconstriction and loss of peripheral NO delivery. Interruption of this cycle by transfusing normal (hemoglobin A-containing) erythrocytes might improve all the abnormalities.