The influence of intermittent prone positioning on pulmonary gas exchange and parenchymal densities was investigated in ten patients. Three patients fulfilled the criteria of "severe ARDS"; seven patients had moderate lung injury as documented by the "lung injury score" (LIS). Nine out of ten were trauma patients and had an average injury severity score of 32.3. The oxygenation index (paO2/FiO2) improved in all patients after prone positioning, showing an increase of 81 +/- 47 (20-197) mmHg (MV +/- SD). Total static lung compliance showed an increase of 7.6 (0-16) ml/mbar. Patients with ARDS benefitted especially from intermittent prone positioning, resulting in a substantial improvement of the LIS of 0.91 (0.75-1.0) points. In moderate lung injury the LIS improved 0.5 (0.25-1.0) points. In lung computerized tomography, we observed the disappearance of posterobasal densities. Repeated prone positioning may therefore be used in ARDS patients as well as in patients with moderate lung injury.