Correct inflation of the cuff of any breathing tube is important; overinflation can damage the mucosa of the tracheobronchial tree and underinflation will cause a leak. Three different techniques to determine cuff seal/leak during inflation of the bronchial cuff of left double-lumen endobronchial tubes (DLT) were each evaluated in 10 patients. DLT size was chosen from a formula based on the patient's height and sex. In the first technique, designated the positive pressure technique of bronchial cuff inflation (PPT), the bronchial side of the DLT is pressurized during connection of the tracheal side to a beaker of water. Air bubbles will appear in the beaker in the absence of bronchial seal. The second technique, designated the CO2 analysis technique of bronchial cuff inflation (CAT), is based on analysis of CO2 content of gas sampled from the tracheal side of the DLT during ventilation of its bronchial side. When the bronchus is sealed, the normal CO2 waveform changes to a flat line. In the third technique, designated the negative pressure technique of bronchial cuff inflation (NPT), suction is applied to the tracheal side of the DLT. The absence of bronchial seal will result in collapse of the reservoir bag within a breathing system connected to the bronchial side of the tube. The bronchial sealing volumes were 1.1 ± 0.9 mL, 0.9 ± 0.7 mL, and 0.3 ± 0.4 mL (mean ± SD) when measured with the PPT, CAT, and NPT, respectively. The cuff volumes were significantly smaller when measured with the NPT compared to the PPT and CAT (P < 0.01) and when measured with the CAT compared to the PPT (P < 0.05). Lung collapse was satisfactory in all patients when the cuff volume corresponded to the sealing volume measured with the NPT, which was the smallest cuff volume measured. In 6 of the 10 patients, satisfactory lung collapse was achieved without any air in the cuff. These results suggest that when an appropriate-sized endobronchial tube is used, satisfactory lung collapse can be produced with little or no air in the bronchial cuff. These findings, however, do not define the best method of seal that prevents spread of blood or secretions.