Over a 7-year period, 15 pregnant women admitted to Parkland Memorial Hospital for acute pyelonephritis developed respiratory insufficiency characterized by dyspnea, tachypnea, hypoxemia, and radiographic evidence of pulmonary infiltrates. Clinical manifestations usually appeared 24 to 48 hours after the patient was admitted and varied from mild respiratory distress to pulmonary failure in three; these three required tracheal intubation and mechanical ventilation. We found no evidence that pulmonary edema was caused by intravenous fluid overload. Oxygen therapy and ventilation were given to maintain the arterial PO2 at 80 mm Hg or greater, and erythrocyte transfusions were given to six women to correct anemia. Women with pulmonary injury were more likely to have multisystem derangement than a control group without respiratory involvement, but there were no clinical risk factors that were predictive at admission. This syndrome was probably caused by permeability pulmonary edema, likely mediated by endotoxin-induced alveolar-capillary membrane injury since other evidence of endotoxemia was common. Thrombocytopenia, hemolysis, intravascular coagulation, renal dysfunction, and transient cardiomegaly concomitant with hyperdynamic ventricular function are all explicable from endotoxin effects.