Background <p>The upper airway morphology in children varies with age and body position. This study aimed to analyze the impact of lateral positioning on the upper airway of sedated children under five.</p> Methods <p>This retrospective study included pediatric patients who underwent MRI in both the supine and lateral positions at Children’s Hospital, Zhejiang University School of Medicine. Upper airway morphology was reconstructed using 3D Slicer software. Python was employed to estimate cross-sectional areas via pixel analysis. The narrowest cross-sectional area, minimal transverse and anteroposterior diameters, airway length, and airway volume were measured and stratified by age for subgroup analysis.</p> Results <p>In sedated children under 5 years old&#xa0;and when compared to the supine position, lateral positioning increased minimal transverse diameter by 18.70% (<i>P</i> = 0.001), narrowest cross-sectional area by 49.21% <i>(P</i> &lt; 0.001), anteroposterior diameter by 25.54% (<i>P</i> &lt; 0.001), airway volume by 65.64% (<i>P</i> &lt; 0.001), and airway length by 11.93% (<i>P</i> &lt; 0.001). In all subgroups, lateral positioning significantly increased the narrowest cross-sectional area, airway length, and airway volume. However,&#xa0;minimal anteroposterior diameter in the 1-to 3-year age group and minimal transverse diameter in the 3 -to 5-year age group tended to increase in the lateral position but did not reach statistical significance.</p> Conclusions <p>Lateral position significantly enlarges the upper airway in sedated children under five. These findings support using lateral position to enhance airway patency in younger patients.</p> Graphical abstract <p></p>