Plasmatic immunoreactive trypsin (IRT), thromboxane and trypsin-like enzymatic activity were measured in 117 patients at risk of developing adult respiratory distress syndrome (ARDS) (53 multiple injury, 30 abdominal surgery, 17 acute pancreatitis, 12 burnt and 5 disseminated intravascular coagulation patients). 69 of these patients developed ARDS. Immunoreactive trypsin and thromboxane were measured by radio-immuno-assay and trypsin-like enzymatic activity by spectrophotometry, using a specific chromogenic substrate. Mean IRT value was 675 ng/ml in ARDS and 265 ng/ml in non ARDS patients (p less than 0.05). Mean IRT value was 685 ng/ml in septic and 170 ng/ml in non septic patients (p less than 0.01). An abnormal trypsin-like enzymatic activity was measured in 26 ARDS patients. In 60 patients (37 ARDS and 23 non ARDS), thromboxane appeared in plasma simultaneously or about 24 hours after the beginning of IRT release. The importance of thromboxane release parallels the intensity of IRT. Originating from pancreas, trypsin can appear in plasma either by absorption from gastrointestinal tract or after pancreatic ischemia.