The pneumatic anti-shock garment is a widely used prehospital treatment modality for injured patients in shock. Laceration of the hemidiaphragm is a post-traumatic condition that usually is unrecognized during the prehospital phase of trauma care. In order to study the effects of inflation of the pneumatic anti-shock garment on experimental swine with diaphragmatic injury, a premeasured custom-fitted external counter pressure device was applied to the swine both before and after surgically induced laceration to the left hemidiaphragm. At each inflation pressure, several measurements were recorded: blood pressure, heart rate, respiratory rate, tidal volume, pulmonary artery pressures, pulmonary capillary wedge pressure, cardiac output and arterial blood gas determinations. Roentgenograms of the chest were obtained both before and after diaphragmatic laceration and at each inflation pressure after injury. Herniation of abdominal viscera was induced in all swine at 60 millimeters of mercury and was accompanied by statistically significant aberrations in tidal volume, pH, pO2, pCO2, pulmonary capillary wedge pressure and pulmonary artery pressures. Blood pressure increased, in both the injured and noninjured swine, and remained at normal or above normal levels throughout the inflation period in both groups. Extrapolating this phenomenon to the clinical setting when the use of or further inflation of the pneumatic anti-shock garment, or both, is generally determined by blood pressure response alone, patients with blunt trauma and lacerated diaphragms in whom external counter pressure is used may sustain irreversible cerebral hypoxia before any problem is suspected.