With the purpose of studying the role of proteinases in the development of ARDS, plasma levels of immunoreactive trypsin (IRT) and amylase were measured in 43 intensive care patients at risk of developing ARDS (22 polytrauma, seven abdominal surgery, four burns, two DIC and eight pancreatitis). Twenty four of these 43 patients developed ARDS and 31 presented abnormal IRT values (above 70 micrograms/L). Twenty-one of these 31 patients had ARDS; a significant correlation thus appeared between ARDS and abnormal IRT values. In nine patients, IRT values were higher than 800 micrograms/L and remained high for 3 to 4 days. A statistically significant correlation also appeared between abnormal IRT and septic phenomena: 20 patients with high IRT values presented septic problems. When IRT values were high, amylase values were often also abnormal: 12 of 23 patients with high IRT had abnormal amylase levels (the eight patients with documented pancreatitis were excluded); no other clinical signs or symptoms of pancreatitis were present in these patients. IRT could be one of the mediators of ARDS in septic patients. It is not clear that the pancreas is the origin of IRT in all cases.