Five mechanically ventilated patients were studied during hemodialysis. The aim was to determine if hypoxemia would develop, and to identify the causes. Respiratory variables (dynamic compliance, peak airway pressure, CO2production); oxygen uptake, and transport variables (alveolar and arterial PO2, pulmonary venous admixture, oxygen consumption); respiratory quotient; pulmonary vascular resistances and white blood cells (WBC) were measured. PaO2 decreased during dialysis, as did PaO2. However, the fall in alveolar oxygen tension failed to explain the hypoxemia. Lung volume did not change significantly, because dynamic compliance, peak airway pressure, and pulmonary vascular resistance were not modified. CO2 losses through the dialysis coil were of little clinical significance. WBC count fell significantly. The authors conclude that ventilation/perfusion and diffusion abnormalities related to leuko-agglutination are responsible for hypoxemia during dialysis.