Clearance of inhaled 99mTechnetium-labelled diethylene triamine pentacetate (99mTc-DTPA) from the lung, an index of pulmonary alveolar epithelial permeability (PAEP), was measured in 13 patients with cardiogenic interstitial pulmonary oedema (CIPO) and in 7 patients with adult respiratory distress syndrome (ARDS). Thirty-five normal subjects (22 nonsmokers and 13 smokers) were evaluated as controls. Half-time clearance (t0.5) values in ARDS patients (mean +/- SD: 15 +/- 2 min) were significantly lower than in CIPO patients (62 +/- 9 min). This PAEP increase in ARDS was impressive, even in comparison to heavy smokers. Loss of the PAEP vertical gradient (apical PAEP greater than base PAEP) was observed in both cardiogenic and ARDS lungs and among smokers.