A new experimental model is described in which pulmonary changes identical with the adult respiratory distress syndrome (ARDS) can be induced by reproducible musculoskeletal trauma in anesthetized pigs. The pigs were studied in maintained anesthesia for 3 days after the trauma under standardized conditions. The intrapulmonary aggregation of platelets and fibrin was monitored by external detection of radioactivity arising from pretrauma intravenous injection of 51Cr-labeled platelets and 125I-labeled fibrinogen. Pulmonary trapping of platelets and fibrin was significantly greater in the traumatized pigs than in nontraumatized but otherwise identically handled controls. Radiologic and morphologic changes corresponding to ARDS developed in the traumatized animals, but not in the controls. The experimental model offers new possibilities for study of factors influencing the occurrence and development of ARDS. After further experimental evaluation, the procedure for registering pulmonary microembolism by external detection may be useful in the clinical management of ARDS.