The extended focused assessment with sonography for trauma (eFAST) ultrasound examination is an essential step in the initial assessment of trauma patients. Its accuracy depends on the ability to acquire high-quality ultrasound images, and we hypothesized that increasing BMI was associated with increased odds for incorrect eFAST. All adult blunt trauma activations at a high-volume urban trauma center in 2016 that underwent eFAST and CT chest, abdomen, and pelvis were included (n = 446). Sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of the eFAST were calculated with CT results as reference. The association of BMI and eFAST accuracy was determined using univariate analyses. Sensitivity and specificity of the eFAST examination were 27.1 per cent and 91.7 per cent, respectively, with an overall 76.2 per cent accuracy. At BMI 36 kg/m², the odds of having incorrect eFAST results increased to odds ratio (OR) = 1.85 (95% confidence interval, 1.03-3.32; P = 0.05). For those with BMI > 40 kg/m², the OR increased to OR = 3.12 (95% confidence interval, 1.45-6.69; P = 0.01). One-third of patients in this study were obese or morbidly obese. The latter was associated with increased odds for incorrect eFAST results, particularly the abdominal examination component.