Determination of appropriate sized endotracheal tube is essential in children. Intubation of pediatric patients with an endotracheal tube (ETT) that is too small may result in insufficient ventilation, poor reliability of end-tidal gas monitoring, leakage of anesthetic gases into the operating room environment, and an enhanced risk of aspiration. In contrast, an ETT that is too large can cause upper airway damage (e.g., local ischemia, ulceration, scar formation) and the potential for subsequent subglottic stenosis. Age-based formulas are commonly  used to estimate optimal ETT size.  Predictive formulas for appropriate ETT size have also been based on patient weight and height. However, none of these formulas are able to determine the appropriate size of the endotracheal tube with great accuracy. Studies have shown that agreement rate of these formulas for determining the appropriate endotracheal tube size in paediatric patients is as low as 47-77%.  The result is that repeated laryngoscopies are often necessary to identify the appropriate tube for individual patients. Recent reports suggest that the diameter of the subglottic upper airway can be determined by ultrasonography in young adults and pediatric patients. A new regression formula based on measurement of subglottic diameter by ultrasonography has been formulated by Shibasaki et al. Unlike the age based formulas , not many studies have been done to evaluate the  agreement rate of the endotracheal tube size determined clinically  and that determined by  formula based on ultrasonography.