Fluid management in the critically ill patient receiving high levels of positive end-expiratory pressure (PEEP) can be difficult. PEEP may cause the cardiac index to fall due to a decrease in left ventricular preload. However, the high intrathoracic pressures produced by PEEP negate the usefulness of the pulmonary artery occlusion pressure (PAo) as a measurement of left ventricular preload. The military antishock trouser (MAST), which has been presumed to compress the venous capacitance reservoir and auto-transfuse 500 to 1,000 ml to the central circulation, was used as a reversible predictor of the effects of fluids on 12 critically ill patients receiving PEEP greater than 10 cm H2O with a decreased cardiac index. Hemodynamic variables were measured before, during, and after MAST inflation. Fluids were given in a quantity sufficient to maintain the same PAo after MAST deflation as achieved with the initial inflation. A significant improvement of cardiac performance and a high correlation between MAST and post-MAST variables was observed. Application of MAST as a reversible fluid challenge is a useful method for predicting optimal fluid management.