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In this study, we evaluated the ratio of the outer diameter of an uncuffed Mallinckrodt endotracheal tube (ETT, Mallinckrodt Tyco Healthcare UK Ltd) to the MRI-measured internal transverse airway diameter (ITD) at the cricoid level in children.With institutional ethics committee approval and parental consent, we measured the ITD at the cricoid level from MRI images of children undergoing MRI diagnostic procedures requiring general anaesthesia with orotracheal intubation. The ITD at the cricoid level was compared with the outer diameter of the utilized ETT. Linear regression analysis was employed to assess the correlation of tracheal diameter with age, height and weight. For all tests, a P value of less than 0.05 was considered to be statistically significant.Fifty patients were studied (21 girls and 29 boys). The difference between the ITD at the cricoid level and the outer tracheal tube diameter ranged from 0.1 to 1.7 mm (median 0.7 mm). The internal transverse tracheal diameters had a strong association with age (r2 = 0.7077, P < 0.001), a moderate association with height (r2 = 0.5928, P < 0.001), and a mild association with weight (r2 = 0.2437, P < 0.001).The outer diameter of the ‘best-fit’ ETT was less than the ITD at the cricoid level by 0.1–1.7 mm. The correlation of the outer diameter of the ‘best-fit’ ETT with age was stronger than with height or weight.