Impairment of alveolar surfactant function has been documented in the acute respiratory distress syndrome (ARDS) and in severe pneumonia (PNEU); however, the underlying mechanisms are not completely understood. In the current report we present a detailed analysis of fatty acid (FA) profiles of different surfactant phospholipid (PL) classes isolated from bronchoalveolar lavage fluids (BALF) and large surfactant aggregates (LSA) from mechanically ventilated patients with ARDS (n = 8), ARDS associated with lung infection (ARDS + PNEU, n = 9), and PNEU (n = 22). Healthy volunteers served as control subjects (n = 8). PLs were isolated by thin-layer chromatography, and the FA profile of each PL class was assessed by gas chromatography. In addition, the minimal surface tension (gamma min) of untreated LSA and of LSA after supplementation with additional dipalmitoylated phosphatidylcholine (DPPC) was analyzed (pulsating bubble surfactometer). As compared with control LSA, the percentage of palmitic acid in phosphatidylcholine (PC) was significantly decreased in all patient groups (ARDS 63.0 +/- 2.0%, ARDS + PNEU 64.6 +/- 4.9%, PNEU 65.6 +/- 1.5%, control subjects 80.1 +/- 1.7%), whereas the relative amount of unsaturated species in PC increased significantly in all groups. Phosphatidylglycerol (PG) and phosphatidylinositol (PI) presented similar FA profiles in control subjects, but differed in the patients. The FA pattern of sphingomyelin (SPH) and phosphatidylethanolamine (PE) displayed only minor changes under conditions of respiratory failure. As compared with control subjects a highly significant increase of gamma min from near zero to approximately 16 mN/m was observed in all patients and was found to be inversely correlated to the percentage of palmitic acid in PC of LSA or BALF. Accordingly, values for gamma min were significantly improved upon secondary supplementation of LSA with DPPC up to control values. We conclude that marked changes in the FA composition of the predominant surfactant PL classes occur, both in ARDS triggered by nonpulmonary events and PNEU. The marked reduction of palmitic acid in the PC fraction may be related to changes in surfactant function under these conditions.