BACKGROUND: Side impact motor vehicle collisions (MVCs) are associated with higher morbidity and mortality compared with other types of MVCs. The stiffness of the lateral aspect of the vehicle and restraint use may play a role. The purpose of this study was to evaluate the role of restraint use, vehicle size, and compartment intrusion on the incidence of splenic injury in side impact MVCs. METHODS: The National Automotive Sampling System was used to identify drivers involved in side impact collisions for the years 1996 to 1998. The incidence of splenic injury in these collisions was compared according to restraint use, vehicle size, and magnitude of vehicle crush. Information on the perceived cause of splenic injuries sustained in the MVC was also obtained from National Automotive Sampling System investigator records. RESULTS: Overall, among drivers involved in side impact MVCs, restraint use was associated with a significantly reduced rate of mortality (odds ratio [OR], 0.40; p < 0.0001) and splenic injury (OR, 0.76; p < 0.0001). Restrained drivers of small vehicles (<2,500 lb), however, had a higher incidence of splenic injury in both minimal (lateral intrusion < 30 cm) (OR, 60.1; p < 0.0001) and severe (lateral intrusion > 30 cm) (OR, 4.0; p < 0.0001) magnitudes of vehicle crush on the driver's side of the vehicle. For both midsize (2,500-3,000 lb) and large (>3,000 lb) vehicles, restraint use was associated with a lower risk of splenic injury regardless of the magnitude of crush. In nearly all cases of splenic injury, the left vehicle interior was the source of injury. CONCLUSION: Overall, restraint use is associated with lower rates of splenic injury and mortality in side impacts. Despite this fact, restrained drivers of small vehicles have a higher risk of splenic injury after lateral impact MVCs when compared with unrestrained drivers. Evaluation of the combined role of restraint use, crash, and injury patterns may provide novel insight regarding vehicle safety design features.