BACKGROUND: The manufacturer has recommended that 6-7 ml of air be used to seal the bronchial blocker cuff of Univent tubes high pressure type. However overinflation of the cuff might result in pressure damage to mucosa. This study was performed to evaluate the appropriate sealing volume and pressure of the bronchial blocker cuff. METHODS: Univents were inserted in 26 patients. The bronchial blocker was connected to a pressure gauge and the cuff was inflated by 0.5 ml increment until the bronchus was sealed in supine and lateral positions under 20-25 mmHg airway pressure. The sealing volume and pressure were recorded. RESULTS: In males, the sealing volume for the left bronchus (3.8 +/- 0.4 ml) was smaller than for the right bronchus (5.4 +/- 1.0 ml) in both positions significantly (p < 0.05). In females, the sealing volume for the left bronchus (2.8 +/- 0.4 ml in both positions) was smaller than the volume for the right bronchus (3.7 +/- 0.8 ml in supine position, 3.9 +/- 0.9 ml in lateral position) significantly (p < 0.05). The sealing volume for females was smaller than for males significantly (p < 0.05), for all sealing pressures, there were no significant differences statistically. CONCLUSIONS: The bronchial blocker of Univent is a high pressure type and the sealing volume is higher for the right bronchus than for the left bronchus. Furthermore the sealing volume is less than the volume which is recommended by the manufacturer.