In Korea, the study about the normal depth of insertion of dauble lumen tubes bas not been reported yet. So we studied 50 adult patients(30 males and 20 females) undergoing thoracic surgery under one lung ventilation. Endotracheal intubation was done with the disposable polyvinylchloride Robertshaw-type double lumen endobronchial tube. The exact position of a left-sided double lumen endobronchial tube was confirmed by passing a fiberoptic bronchoscope through the tracheal lumen of the double lumen, The depth was measured when we had a view of a clear straight-ahead view of the tracheal carina, the right lumen going off to the right, and the upper surface of the blue left endobronchial balloon just below the tracheal carina, The resullts were as follows; 1) In the male patients, the average depth of the tube was 30.7±1.4cm and this measure- ments were related with height but not the weight and age. 2) In the female patients the average depth of the tube was 28,3±l.2cm and this measurements were unrelated with height, age and weight. 3) Among the male and female patients the average depth of the tube was 30.0±1.8cm and this measurements were related with a height but not with weight and age.