The left main stem bronchus is more horizontal than the right. It generally takes off at a 40 to 50° angle with respect to the trachea. It is also longer (5 cm), but smaller in diameter (12 mm) than the right main stem bronchus.

It bifurcates into the left upper lobe bronchus which runs outward, forward, and upward along its anterolateral aspect, and the lower lobe bronchus.