<p>The impact of body mass index (BMI) on the outcomes of head and neck free flap reconstructions, the gold standard method for treating complex defects, is not well-established. Understanding this relationship is essential for improving preoperative patient optimization. Studies investigating the association between BMI and outcomes of head and neck free flap reconstruction were identified from systematic searches of the PubMed, EMBASE and CENTRAL databases. The quality of evidence was assessed using the Newcastle-Ottawa Scale. Due to study design variability, a narrative synthesis was employed rather than a meta-analysis. 63 studies from 15 countries, encompassing 28,008 patients, were included. These studies were a mix of 34 cohort and 29 case-control studies. Underweight patients (BMI &lt; 18.5&#xa0;kg/m²) had reduced survival rates (5 out of 6 studies), higher transfusion requirements (2 out of 4 studies), and greater post-hospital rehabilitation needs (2 out of 2 studies). Obese patients (BMI &gt; 30&#xa0;kg/m²) showed an increased risk of venous thromboembolism (3 out of 4 studies) and postoperative pulmonary complications (2 out of 3 studies). No significant associations were found between BMI and other outcomes like flap failure, surgical site infections, unplanned readmissions, length of stay, and functional outcomes. The findings indicate that underweight patients may be at higher risk of reduced survival, increased transfusion needs, and more postoperative rehabilitation. Conversely, obese patients face heightened risks of venous thromboembolism and pulmonary complications. This study offers valuable insights into enhancing patient risk stratification and optimisation before head and neck free flap reconstruction.</p>