•‘The Obesity Paradox’ holds true for pelvic and acetabular fractures: low BMI is associated with increased mortality in patients sustaining a pelvic or acetabular fracture. Fractures of the pelvis and acetabulum are often the consequence of high energy trauma in young individuals or fragility fractures in osteoporotic bone. They can be life-threatening or life changing injuries. No published data exists comparing body mass index (BMI) and mortality for this patient group. The aim of this study was to identify if low BMI (<18.5) was a predictor of morbidity and mortality for patients with these injuries. Of the 1033 patients with pelvic or acetabular fractures referred to a single level 1 major trauma centre (MTC) over a 4.5-year period (August 2015 - January 2020); we retrospectively analysed data for all admitted patients. Data was collected on demographics, injury pattern, operative intervention and complications. Comparison was made between patients that were underweight (BMI<18.5) and patients that were not. Both in-hospital and post discharge complications were recorded including pulmonary embolus (PE), deep venous thrombosis (DVT), ileus, infection, loss of reduction and mortality at 6 months. 569 patients admitted to the MTC with a pelvic or acetabular fracture were included in our analysis. Underweight patients had a statistically significant increase in mortality both in-hospital (p = 0.019) and at 6 months post injury (p = 0.039) when compared to other BMI groups. No statistical significance was found between these BMI groups comparing morbidity: DVT (p = 0.712), PE (p = 0.736) nor ileus (p = 0.149). Covariate analysis showed that a low BMI was associated with triple the in-hospital mortality after correction for age and energy of injury (adjusted OR 3.028, 95% CI 1.059-8.659). This is the first published study that demonstrates a statistically significant increase in mortality in patients with pelvic or acetabular fractures who are underweight. Surgeons should carefully consider appropriate peri-operative optimisation for these patients. Further investigation into the effects of low BMI and response to trauma is required.