We assessed hemodynamics, total lung and chest wall compliance (CT) and gas exchange using two different levels of PEEP during controlled ventilation in two different groups of patients with ARF; in the first group (Group 1, 12 patients) chest X-Rays showed a symmetrical pattern of bilateral alveolar infiltrates; in the second group (Group 2, 5 patients) chest X-Ray showed a asymmetrical pattern with unilateral lobar consolidation. A first level of PEEP (best PEEP = 9 +/- 3 cm H2O) produced an improvement in CT and in gas exchange with a slight decrease in cardiac index in both groups; but improvement in PaO2 (from 64 +/-33 to 122 +/- 76 torr, p less than 0.001 in Group 1, and from 76 +/- 39 to 91 +/- 33 torr, p less than 0.05 in Group 2) and decrease in QS/QT were not as well marked in Group 2 as i Group 1. A second level of PEEP (high level PEEP: 20 +/- 4 cm H2O) produced a sharp decrease in CT and required hemodynamic support in each case (blood volume expansion with or without Dopamine infusion) to maintain cardiac index within a normal range. In Group 1 this high level PEEP produced a greater improvement in gas exchange (PaO2 increased from 122 +/- 76 to 194 +/- 76, p less than 0.01) but in Group 2 it had a deleterious effect, producing a decrease in PaO2 (from 91 +/- 33 to 76 +/- 41 torr, p less than 0.05), and an increase in QS/QT; with this higher PEEP we also noted an increase of alveolar dead space in Group 2. This study demonstrates the efficiency of high levels of PEEP to reduce QS/QT in ARF but also shows its limitations: namely reduction in cardiac performance and in efficiency if the damage to one lung is significantly more pronounced than that to the other lung.