An endobronchial blocker catheter (10) for use in blocking a selected mainstem bronchus (65) under guidance of a bronchoscope (64). The endobronchial blocker catheter comprises a tubular assembly and an elongated body that receives the tubular assembly. The tubular assembly comprises a tubular member (40) and a loop (45) disposed at a distal end of the tubular member. The loop is sized such that the bronchoscope is passable through the loop. The elongated body (12) has a plurality of lumens (28, 29, 30) therein, and an inflatable blocker balloon (17) disposed about a distal portion of the elongated body. One of the lumens (28) extends to an interior of the balloon to accomplish inflation of the balloon for retaining the blocker catheter in the mainstem bronchus, and another lumen (29) is sized for receiving the tubular assembly in a manner such that the loop extends from a distal end of the elongated body. The blocker catheter is guided by the bronchoscope to the desired inflation site via the loop.