Background: Seat belts and air bags have been shown to significantly reduce morbidity and mortality following MVCs. Research suggests that restraint use does not protect against lower extremity fracture; however, no population-based studies of this association exist. The purpose of this study is to compare the effectiveness of combined seat belt and airbag restraint systems with airbag alone, seat belt alone, and no restraints with respect to incidence and location of lower extremity fractures. Methods: A retrospective analysis of front seat occupants involved in police-reported, tow-away frontal MVCs was conducted using data from the 1995 through 2000 National Automotive Sampling System (NASS). Incidence and relative risk (RR) of fracture to specific bony regions were measured according to seat belt use and airbag deployment. Results: Compared with unrestrained occupants, occupants restrained with airbag only had significantly higher risk for all types of lower extremity fractures whereas those occupants restrained with either seat belt only or seat belt and airbag had lower risk of fracture. The greatest difference was seen with tibia/fibula fractures in airbag only (RR, 2.14) but this trend continued to be significant with femur and pelvic fractures (RR, 1.13 and 1.23, respectively). Conclusion: While airbags may reduce the risk of death when used alone or in combination with seat belts, the results of this study demonstrate that air bags increase the risk of lower extremity fractures when used as the sole method of passenger protection. Also, they may do so differentially according to skeletal region. This data strongly support the consideration of developing accessory knee bolster airbags to prevent the "submarining" or sliding under the airbag that may be responsible for this finding.