The dysfunction of pulmonary capillary represents the marker of Acute Respiratory Distress Syndrome (ARDS). Each current mean of investigation in ARDS (radiography, CT, ultrasonography, double dilution technique, electric impedance, bronchoscopy) has its downfalls and has not succeeded in becoming a gold standard in evaluation of this pathology. The development of any treatment is hampered by the lack of a rapid and objective method of evaluation of the progress of the disease. The radioisotopic evaluation of pulmonary capillary permeability realizes a picture of a physiologic process blood-interstitial exchange at pulmonary level. The most recent studies appear to suggest that this method of investigation can objectively evaluate and be a predictive parameter for ARDS evolution. We will present the principle of the radio-isotopic method and some of its current clinical applications.