We performed orotracheal intubation in 153 consecutive pediatric patients undergoing cardiac catheterization. Auscultation of bilateral breath sounds was confirmed. By fluoroscopy, the tip of the endotracheal tube (ETT) was seen in the right mainstem bronchus in 18 patients (11.8%) and in a low position, defined as within 1 cm above the carina, in 29 patients (19.0%). All of the 18 patients with right mainstem intubation were children <120 mo of age, and 7 were infants <12 mo of age (Fisher's exact test; P = 0.013). The age, weight, and ETT size for children who had endobronchial and low tracheal positions were significantly (P < 0.001) less than for those who had midtracheal positions. The failure to diagnose mainstem intubation by auscultation alone may be related to the use of the Murphy eye ETT, which reduces the reliability of chest auscultation in detecting endobronchial intubation. Suggested measures for preventing endobronchial intubation include maintaining increased awareness of the imperfection or lack of accuracy of the auscultatory method, assessing insertion depth by checking the length scale on the tube, and minimizing the patient's head and neck movement after intubation. When extreme flexion or extension of the neck is expected after ETT insertion, the resultant change in ETT final position must be anticipated and taken into consideration when deciding on the depth of ETT insertion. This approach resulted in a decrease in improper tube positioning from 20% when the study was initiated to 7.1% in the last 98 patients.