BACKGROUND: Accurate knowledge of mainstem bronchial lengths are required to prevent malpositioning of double lumen endobronchial tubes (DLT). Therefore we evaluated the length of the mainstem bronchus in Korean adults who had no abnormalities in both mainstem bronchus. METHODS: Two-hundred Thirty-seven patients were composed of One-hundred one males and One- hundred Thirty-six females who underwent elective surgery. After an endotracheal tube was placed, we measured the length from the upper incisor to the tracheal carina, the right mainstem bronchial carina, and the left mainstem bronchial carina using a fiberoptic bronchoscope. RESULTS: The lengths from the upper incisor to the carina of a male and female were 26.8 +/- 1.8 cm and 23.6 +/- 1.9 cm respectively, and the correlations between their length and height are significant for male and female (r = 0.32, P < 0.01 and r = 0.56, p < 0.0001). The lengths from the upper incisor to the right mainstem bronchial carina of male and female were 29.0 2.0 cm and 25.3 2.2 cm respectively, and the correlations between their length and height are significant for male and female (r = 0.39, P < 0.0001 and r = 0.59, P < 0.0001). The lengths from the upper incisor to the left mainstem bronchial carina of male and female were 32.0 2.1 cm and 28.5 2.1 cm respectively, and the correlations between their length and height are significant for male and female (r = 0.45, P < 0.0001 and r = 0.60, P < 0.0001). CONCLUSIONS: We found that as the height of patients increased, the length from the upper incisor to the carina, the right mainstem bronchial carina, and the left mainstem bronchial carina increased. Nevertheless,it should be understood that the length of DLT insertion at any given height is still normally distributed, and correct DLT positioning should always be confirmed fiberoptically after the initial placement.