BACKGROUND: Fiberoptic bronchoscopy has been recommended to verify the position of single lumen tubes with bronchial blockers (Univent(R) tube), but this remains controversial. The authors studied the role of a bronchoscopy for placing and monitoring bronchial blockers (BB) after blind intubation and after positioning the patient. METHODS: One hundred patients having thoracic surgery requiring a Univent tube insertion were prospectively studied. After "blind" tracheal intubations with Univent tubes, BB were advanced in the left-side mainstem bronchus for 60 patients and the right-side for 40 patients. A bronchoscopy was performed after conventional clinical verification of correct placement and after patient positioning for a thoracotomy. A BB was considered malpositioned when it had to be moved < 0.5 cm to correct its position. RESULTS: After "blind" BB intubation, clinical evidence of malpositioning was found in 5 patients. This was confirmed by fiberoptic assessment. In 95 patients in whom placement was judged correct by clinical assessment, malpositioning was detected by bronchoscopy in 39 cases. After patient positioning, BB were found to be displaced in 29 patients. Right-side BB were significantly more likely to be malpositioned than were left-side BB. CONCLUSIONS: After blind intubation and patient positioning, more than one third of BB required repositioning. A routine bronchoscopy is therefore recommended after intubation and after patient positioning.