A dual lumen endotracheal tube comprising: a) a main tracheal tube (11, 12) having a proximal end and a distal end and a bronchial tube having a proximal end and a distal end, the bronchial tube extending distally from the distal end of the main tracheal tube (11, 12); b) a main tracheal tube lumen defined by the main tracheal tube (11, 12); c) a flexible dividing wall longitudinally dividing the main tracheal tube lumen into a tracheal lumen and a bronchial lumen each extending from the proximal end of the main tracheal tube to the distal end of the main tracheal tube, wherein a proximal portion of the dual lumen endotracheal tube comprises two extension conduits extending, respectively, the tracheal lumen and the bronchial lumen proximally and each terminating proximally in a respective tube connector (19, 20) for connection to a ventilation or insertion end of a flexible tube and wherein the flexible dividing wall has a neutral position in which it rests during both two- and one lung ventilation; d) the tracheal lumen comprising: a first upper opening at the proximal end of the main tracheal tube; and a first lower opening at the distal end of the main tracheal tube; e) the bronchial_lumen comprising a second upper opening at the proximal end of the main tracheal tube, wherein the bronchial lumen communicates with and is extended distally by a bronchial tube lumen defined by inside walls of the bronchial tube; f) the bronchial tube lumen comprising a third lower opening at its distal end; g) a first inflatable cuff adapted for tracheal sealing, the first inflatable cuff being located on the main tracheal tube proximally relative to the first lower opening of the tracheal lumen; h) a second inflatable cuff (27; 79) adapted for bronchial sealing, the second inflatable cuff being located on the bronchial tube proximally relative to the third lower opening of the bronchial tube; and i) two sealing attachment lines (38, 39) of the flexible dividing wall along the entire length of the inside walls of the tracheal tube (11, 12) that are offset from a midline of the main tracheal tube (11, 12), the dividing wall defining cross sectional areas of the tracheal lumen and the bronchial lumen that are about equal when the dividing wall is in its neutral position, characterised in that: at least a major part of an outside wall of the main tracheal tube (11, 12) is reinforced; and in that: in the neutral position the dividing wall is: semi-lunar in cross-section; and convex to the bronchial lumen and concave to the tracheal lumen. a) a main tracheal tube (11, 12) having a proximal end and a distal end and a bronchial tube having a proximal end and a distal end, the bronchial tube extending distally from the distal end of the main tracheal tube (11, 12); b) a main tracheal tube lumen defined by the main tracheal tube (11, 12); c) a flexible dividing wall longitudinally dividing the main tracheal tube lumen into a tracheal lumen and a bronchial lumen each extending from the proximal end of the main tracheal tube to the distal end of the main tracheal tube, wherein a proximal portion of the dual lumen endotracheal tube comprises two extension conduits extending, respectively, the tracheal lumen and the bronchial lumen proximally and each terminating proximally in a respective tube connector (19, 20) for connection to a ventilation or insertion end of a flexible tube and wherein the flexible dividing wall has a neutral position in which it rests during both two- and one lung ventilation; d) the tracheal lumen comprising: a first upper opening at the proximal end of the main tracheal tube; and a first lower opening at the distal end of the main tracheal tube; e) the bronchial_lumen comprising a second upper opening at the proximal end of the main tracheal tube, wherein the bronchial lumen communicates with and is extended distally by a bronchial tube lumen defined by inside walls of the bronchial tube; f) the bronchial tube lumen comprising a third lower opening at its distal end; g) a first inflatable cuff adapted for tracheal sealing, the first inflatable cuff being located on the main tracheal tube proximally relative to the first lower opening of the tracheal lumen; h) a second inflatable cuff (27; 79) adapted for bronchial sealing, the second inflatable cuff being located on the bronchial tube proximally relative to the third lower opening of the bronchial tube; and i) two sealing attachment lines (38, 39) of the flexible dividing wall along the entire length of the inside walls of the tracheal tube (11, 12) that are offset from a midline of the main tracheal tube (11, 12), the dividing wall defining cross sectional areas of the tracheal lumen and the bronchial lumen that are about equal when the dividing wall is in its neutral position, characterised in that: at least a major part of an outside wall of the main tracheal tube (11, 12) is reinforced; and in that: in the neutral position the dividing wall is: semi-lunar in cross-section; and convex to the bronchial lumen and concave to the tracheal lumen.