The use of flexible bronchofiberscopy through endotracheal tubes is limited by the respiratory obstruction it causes in the tube. In the present investigation, airflow resistance was determined in translaryngeal and tracheostomy tubes with and without a flexible bronchofiberscope with an outer diameter of 5.4 mm. In tubes with a bore diameter exceeding 8.0 mm, the resistance increase by the bronchofiberscope was found to be so small, that endoscopy could be carried out without any precautions in most cases. In tubes with a bore diameter less than 8.0 mm, the bronchofiberscope caused high resistance to flow. Measures that reduce the risks of bronchofiberscopy at increased flow resistance are discussed.