Fourteen anesthetized swine were subjected to blunt lateral impact at velocities of 4.3, 6.7, or 8.2 m/s with a 15 cm flat pendulum weighing 23.4 kg accelerated to impact speed by a power-assisted pneumatic impactor. Injuries consisted of laceration of the liver and spleen resulting in severe hemoperitoneum and death by ventricular fibrillation and respiratory arrest in the highest severity impacts. Logist analysis of the biomechanical responses and serious or fatal injury indicated that the maximum Viscous response (VC) had the best correlation with injury risk. A tolerance level of VC = 0.89 m/s was determined for a 25% probability of serious injury. In contrast, maximum chest compression did not correlate with injury. The experiments indicate that internal organ and soft tissue injury may occur by a Viscous mechanism during the rapid phase of compression of the body. The Viscous response is an effective measure of injury risk in side impacts.