3 patients suffering from alveolar proteinosis were treated with massive pulmonary lavage. The lavage method followed the technique described by Ramirez-R, in which one lung was degassed and irrigated repeatedly by liters of a warmed saline solution. During lung lavage, percutaneous and intra-arterial PO2 electrodes were used to monitor the blood gas changes. Swan Ganz catheter and systemic arterial pressure line were placed to monitor the hemodynamic changes. Arterial oxygen tension increased when the lung was filled with the solution, whereas PaO2 decreased immediately when the solution was drained out of the lung. Percutaneous and intra-arterial PO2 electrodes traced these changes very clearly. Pulmonary arterial, central venous and pulmonary capillary wedge pressures were increased at filling into the lung and were decreased at emptying, whereas cardiac output measured by thermo-dilution method was decreased at filling and was recovered at emptying. These hemodynamic and blood gas changes were mainly explained by the changes of pulmonary circulation. The application of positive end expiratory pressure (PEEP) to the ventilated lung suppressed the value of PaO2, therefore PEEP application was considered to be contraindicated. Besides the continuous monitoring of electro-cardiogram and systemic arterial pressure, percutaneous PO2 monitoring seems to be very valuable for the establishment of safety pulmonary lavage.