Monitoring gastric residual volume (GRV) and holding or interrupting enteral nutrition for large or elevated GRV has had a firm place in the recommendations of critical care nutrition guidelines. The present study aimed to assess the feasibility of measurement and the influence of critical illness on ultrasound-measured gastric CSA. 46 mechanically ventilated patients and 30 older adults were included in this prospective trial. All patients were subjected to careful history taking, thorough clinical assessment, and standard laboratory investigations. In all patients, CSA was measured by ultrasound at the first 24h of admission and compared to age and sex-matched healthy controls. Patients had significantly larger CSA in comparison to controls [median (IQR): 1.1 (0.9 to 1.5) versus 0.7 (0.5 to 1.0)]. Patients with septic shock diagnosis had the highest gastric CSA. At a cut-off of 0.77, CSA has good predictive power for critical illness [AUC (95% CI): 0.72 (0.56 – 0.88)] with a sensitivity and specificity of 81 % and 60 %. The current study suggested gastric size is significantly affected in critically ill children and this needs to be considered when deciding when to start enteral nutrition.