BACKGROUND: The aim of the present study was to detect and quantify auto-positive end-expiratory pressure (auto-PEEP) in anesthetized patients using a Laser-Flex endotracheal tube (Mallincrodt, ID, 6.0 mm), by comparing the effects of changes in tidal volume and respiratory rate. METHODS: All patients (n = 30) undergoing elective surgery were anesthetized, paralyzed and intubated with a ID 8.0 mm endotracheal tube (n = 10, control), ID 6.0 mm endotracheal tube (n = 10, group S), or ID 6.0 mm Laser-Flex endotracheal tube (n = 10, group L), respectively. After anesthetic induction, ventilator settings using a Siemens Servo 900C were changed for a tidal volume of 8, 10 ml/kg, respiratory rates of 10, 12 or 14 breaths/min. Peak airway pressure was measured and auto-PEEP was quantified using an end-expiratory occlusion method. Data recorded on the Bicore CP-100 pulmonary monitor was transfered to a PC and analyzed by processing software (ANADAT). RESULTS: In group S and L, peak airway pressure and auto-PEEP were higher than control group and increased during an increase in tidal volume (P < 0.05). But they were increased significantly during an increase of respiratory rate, only when the tidal volume was 10 ml/kg (P < 0.05). CONCLUSIONS: There was an increase of auto-PEEP in anesthetized patients using a Laser-Flex endotracheal tube during incremental changes of tidal volume and respiratory rates.