Data on the normal depth of insertion of double-lumen tubes have not been published. We studied 101 adult patients undergoing thoracic operations whose tracheas were intubated with a left double-lumen tube. A fiberoptic bronchoscope was introduced into the tracheal lumen, and the tube position was adjusted until the cephalad surface of the bronchial cuff was immediately below the carinal bifurcation. The average depth of insertion for both male and female patients 170 cm tall was 29 cm, and for each 10-cm increase or decrease in height, average placement depth was increased or decreased 1 cm. The correlation between depth of insertion and height was highly significant (P less than 0.0001) for both male and female patients. As depth of DLT insertion at any given height was normally distributed, a technique to confirm correct double-lumen tube position always should be used after initial placement.