Objective. To investigate the kinetic and reversibility of mechanical ventilation-associated pulmonary and systemic inflammatory response in patients with acute lung injury (ALI). Design. Prospective observational cross-over study. Setting. Intensive care unit of a university hospital. Patients. Twelve mechanically ventilated patients with ALI. Interventions. Mechanical ventilation was transiently changed from a lung protective setting with PEEP of 15 cmH2O and a VT of 5 ml/kg predicted body weight to a more conventional ventilatory setting with PEEP of 5 cmH2O and VT of 12 ml/kg predicted body weight for a period of 6 h. Measurements and results. We examined the profile of interleukin (IL)-1β, IL-1 receptor antagonist, IL-6, IL-10, and tumor necrosis factor in the plasma of all patients, and in the bronchoalveolar lavage (mini-BAL) fluid of six of these patients. Measurements were performed at baseline, 1 h, and 6 h after each change of the ventilatory setting. Switching to conventional mechanical ventilation was associated with a higher PaO2 (P P Conclusions. In patients with ALI, initiation of low PEEP and high VT mechanical ventilation is associated with cytokine release into circulation which occurred within 1 h. It is independent from BAL procedures and can be reversed by reinstitution of lung protective mechanical ventilation.