The purpose of this study was to investigate the relation between serum lipase (LP), serum immunoreactive trypsin (IRT), and its inhibitors in patients with adult respiratory distress syndrome (ARDS) of diverse origin and to compare their time course with other acute conditions. The IRT and LP levels were determined at regular intervals in 41 patients hospitalized in the intensive respiratory unit with ARDS (n = 9), acute pancreatitis (n = 5), shock (n = 9), bronchopneumonia (n = 10), or acute cardiogenic pulmonary edema (n = 8). Several trypsin inhibitors were measured simultaneously: serum trypsin inhibitory capacity (TIC), alpha 1-antitrypsin, alpha 2-macroglobulin, and antithrombin III. Concomitantly, angiotensin-converting enzyme (ACE) activity was determined as a potential marker of the endothelial injury. A respective 19- and 13-fold increase in IRT and LP values were observed in patients with ARDS after a mean evolution of 6 days; similar increases were seen in patients with pancreatitis. These values were significantly higher than those observed in the other conditions studied. In patients with ARDS and acute pancreatitis, the evolution of IRT and LP values were associated with a sixfold rise in TIC. A low TIC/IRT ratio in patients with ARDS appeared to be an index of poor prognosis. Conversely, ACE activity evolution was characterized by an early decrease in all the conditions studied. These observations indicate that there is an acute delayed pancreas injury in ARDS. Thus, the release of pancreatic enzymes are not reliable markers of the early evolution of the disease but they may represent secondary mediators for enhancement of the increased endothelial permeability.