Both short and longterm effects of positive end-expiratory pressure (PEEP) on oxygenating capacity (OC) were investigated in three groups of patients with acute respiratory failure following multiple trauma (MT). Group A consisted of six patients with “uncomplicated” MT; Group B, eight patients with MT and generalized sepsis; Group C, nine patients with MT and lung contusion. OC was evaluated in terms of PaO2/FIO2 and P(A-a)DO2 on FIO 2 = 1.0. OC was markedly and equally reduced in the three patient groups before use of PEEP. The use of a mean PEEP of 6-7 cm H2O resulted in an initial improvement in mean PaO2/FIO2 of 152.5, 36.1, and 59.2 mm Hg, and an overall improvement of 196.8, 57.5, and 107.0 mm Hg in Groups A, B, and C, respectively. There was a similar improvement in both the initial and the overall effect of PEEP on P(A-a)DO2 in the three groups. The difference in the improvement in OC due to PEEP was statistically significant between Groups A and B. It is concluded that acute respiratory failure following MT includes a wide spectrum of clinical syndromes, and that the improvement in OC due to PEEP depends on the clinical syndrome that is responsible for the respiratory failure associated with MT.