Exhaled breath condensate (EBC) contains small amounts of protein leaving the lung by aerosol droplet generation. Protein patterns in EBC might be useful in monitoring acute and severe pulmonary disease and in particular monitoring of mechanical stress during ventilation. EBC (10ml) was collected from 30 ventilated patients with respiratory failure including 24 patients with acute lung injury/acute respiratory distress syndrome (ALI/ARDS) and from 10 healthy volunteers. Samples were analyzed using gel electrophoresis. Bands were characterized by matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF). In the EBC of mechanically ventilated patients 53.3% exhibited three bands (50-70kDa), 26.7% two bands, 10% one band, and 10% had no bands. While no bands were detected in volunteers EBC. MALDI-TOF analysis identified these bands as cytokeratins 2, 9 and 10. Cytokeratins 2 and 10 were confirmed by Western blot. The detection rate of cytokeratins was correlated to peak inspiratory pressure, positive endexpiratory pressure and ARDS score, but not with inflammatory markers or smoking status. Cytokeratins are present in EBC of mechanically ventilated patients. A strong correlation with parameters of ventilatory stress, such as increased distension, presence of lung injury and time of ventilation suggests a relation with ventilator-associated damage to the pulmonary parenchyma.