The flow in the human trachea is turbulent. Thus, the tracheal resistance (R) and the pressure gradient (ΔP) required to maintain a given flow across the trachea is inversely related to its radius raised to the fifth power. If the caliber reduction ratio (X) after endotracheal intubation is calculated as X = radius of the endotracheal tube (rETT)/radius of the trachea (rT), then ΔP and/or R will be increased by (1/X)5. To measure the actual ratio between rETT and rT following endotracheal intubation of pediatric patients with respiratory failure and to calculate the resulting increase in the tracheal R and ΔP for a given inspiratory flow rate. Retrospective chart review. Pediatric ICU in a tertiary-care teaching children's medical center. Twenty consecutive pediatric patients (mean ± SD age, 6.4 ± 7.2 years) whose tracheas had been intubated for various causes of respiratory failure, and who had received a CT scan, were included in our study. All patients received an endotracheal tube the size of which was derived from the following formula: (age in years/4) + 4. rT and rETT were measured from CT scan sections at and around the level of the thoracic inlet, and the average values were used to calculate X. These values ranged from 0.33 to 0.65 (mean, 0.55 ± 0.8). The factor (1/X)5 was calculated for each patient and then was multiplied by the known normal value for tracheal R for adolescents and adults (0.07 cm H2O/L/s) to obtain the value of R resulting from the artificial airway, (1/X)5 × 0.07. Our results showed that tracheal R increased due to caliber reduction of the trachea after endotracheal intubation by 33.9 ± 52.5-fold (range, 8.6- to 255.5-fold). In order to maintain an inspiratory flow of 1 L/s, the value of P for the intubated trachea would increase from 0.07 cm H2O to a mean of 2.4 ± 3.7 cm H2O (range, 0.6 to 18 cm H2O). In two of our patients, the rT/rETT ratios were < 0.5 (0.33 and 0.44, respectively); this translated into a more significant increase in the calculated ΔPs, 18 and 4.2 cm H2O, respectively. The common value of X due to endotracheal intubation is between 0.5 and 0.6, which in and of itself results in an increase in R across the intubated trachea up to 32-fold. The calculated increase in P as a result of this is between 2 and 3 cm H2O for adolescents or young adults. The addition of pressure support of at least 3 cm H2O during spontaneous ventilation via an endotracheal tube, which is common practice in pediatric critical care, should alleviate any respiratory distress emanating from the increased R. However, a value for X < 0.5, which was found in 10% of our patients (2 of 20 patients), results in a much higher calculated increase in the pressure gradient and, therefore, a higher level of pressure support is required to overcome this increase.