Many methods are taught and used clinically to determine what size uncuffed endotracheal tube is required for the pediatric patient. The purpose of this study was to compare the effectiveness of two methods of selection used clinically: (1) the traditional age-based (AB) formula; (age in years +16) divided by 4, and (2) the method based on body length using the Broselow pediatric resuscitation tape. Following institutional review board approval, 174 patients were prospectively studied after informed consent was obtained. Uncuffed endotracheal tube size selection was determined by randomly assigning the patient to one of the two groups. The appropriateness of the tube selection was assessed using an audible air leak around the endotracheal tube. No difference was found between the AB group and the resuscitation tape group with respect to selecting the appropriate size of endotracheal tube. Retrospective analysis of all patients found another AB formula that is occasionally used ([age in years +18] divided by 4) to be correct in only 20 (11%) of 174 cases. This was significantly different from the other methods (P < .001). Since the AB formula ([age in years +16] divided by 4) is reliable and easily applied, it appears acceptable for routine anesthesia cases in the pediatric population requiring endotracheal intubation. The AB formula ([age in years +18] divided by 4) should be used cautiously because of the high failure rate. In circumstances in which general information, such as age, is not available and endotracheal intubation is needed, the Broselow tape allows reliable endotracheal tube size identification and should be readily available.