BACKGROUND: Obese victims of blunt force trauma have poor outcomes, often related to an increased incidence of missed injuries. Our purpose was to determine whether patients with an increased body mass index (BMI) who sustain blunt trauma in motor vehicle collisions (MVCs) have an increased risk of diaphragmatic injury, an injury often associated with diagnostic delay. METHODS: The National Automotive Sampling System was used to identify front seat occupants involved in MVCs from 1995 to 2001. BMI was determined for all subjects, and diaphragmatic injury was the outcome of interest. Risk ratios (RRs) and 95% confidence intervals (CIs) were calculated to quantify the association between occupant BMI and DI. RESULTS: When analyzed as an independent risk factor, for all collision types, a BMI >/= 25 was not significantly associated with diaphragmatic injury (RR, 1.25; 95% CI, 0.71-2.20). However, when considering only near-side MVCs, a significant twofold increased risk was observed (RR, 2.03; 95% CI, 1.39-2.96), whereas no association for occupants involved in frontal collisions was observed (RR, 1.05; 95% CI, 0.30-3.74). CONCLUSION: An elevated BMI appears to be associated with an increased risk of diaphragmatic injury in patients involved in near-side MVCs. This association provides insight as to the cause of diaphragmatic injury and may aid clinicians in detecting these often occult injuries.