Unexpected tracheal narrowing was observed in a patient with Duchenne muscular dystrophy during a corrective operation for thoracolumbar scoliosis. As the operating time progressed, peak airway pressure and end-tidal CO₂ increased gradually in the prone position. We found a floppy portion of the trachea using fiberoptic bronchoscopy (FB) in the prone position. We advanced a wire-reinforced tube toward the carina beyond the lesion. This allowed correction of the ventilatory abnormalities. We encountered another patient scheduled for the same operation. We performed FB in advance before the position change and observed a narrowed portion of trachea. We advanced the tracheal tube under FB beyond the pathologic portion and then moved the patient into the prone position. The operation was done successfully without any problems.