There are various formulas and algorithms for the prediction of body weight and appropriate ETT size, in pediatric patients. Body Weight estimation is of paramount importance in pediatrics, especially in emergencies. Predicting the appropriate size of the endotracheal tube saves time, money and reduces complications. The goal of this study was to evaluate the validity of two commonly used formulas for predicting the body weight and the size of the appropriate endotracheal tube, both based on age.353 consecutive pediatric surgical patients aged 2 to 12 years, who required general anesthesia and oral endotracheal intubation were included in this study. Patients were stratified according to their age in two groups: group 2-5 (79 children, 2 to 5 years) and group 6-12 (274 children, 6 to 12 years). At the end of surgery an anesthesiologist, who was not involved in the perioperative treatment, recorded the demographic data and also the size and type of the endotracheal tube used. The prediction of Body Weight (BW) was made according to the following formula: 2-5 y.o.: Weight (kg) = (2 x age in years) + 8 and 6-12 y.o.: Weight (kg) = (3 x age in years) + 7. The formula for calculating the size (size= internal diameter=I.D.) of the endotracheal tube (ETT) was: I.D. for cuffed ETT (mm) = (age / 4) + 3.5 and I.D. for uncuffed ETT (mm) = (age / 4) + 4. For all statistical tests p value <0.05 was considered as statistically significant. In all patients as sum and in both age groups, the predicted body weight was significantly (p<0.05) lower than the actual (measured) weight. In group 2-5y.o, 74.7% of patients received cuffed ETT In group 6-12y.o. 100% of patients received cuffed ETT. In group 2-5 y.o, all patients showed a significantly (p<0.05) lower predicted internal diameter of the ETT, either cuffed or uncuffed, compared to ETT ultimately used. In group 6 -12y.o, there was no statistically significant difference between the predicted and the actually used ETT size. The prediction of body weight in children, by the use of the particular formula, led to underestimation. In children aged 2 to 5 years, the application of the inner diameter calculation of the ETT formula also underestimated the appropriate ETT size. It seems that the traditional age-based formulas often fail to predict the correct ETT size in smaller children which probably does not seem to apply to older children.