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Currently, fiberoptic bronchoscopy (FB) is recommended for correct positioning of double-lumen endobronchial tubes (DLTs) because of the high incidence of malpositions not appreciated by clinical signs. The aims of this study were to assess whether clinical signs allow accurate confirmation of adequate positioning with left red rubber (RR) or polyvinylchloride (PVC) double-lumen tubes and to compare the incidence of malpositions between the two tubes. Another goal was to assess whether these malpositions, not appreciated by clinical assessment, adversely affected outcome. Twenty-one adult patients scheduled for elective thoracic surgery were randomly assigned to the RR (11 patients) or PVC group (10 patients). After endobronchial intubation, the position of the tubes was adjusted until clinically satisfactory lung separation had been achieved. A single investigator performed all the FB and assessed adequacy of tube placement. Clinical and FB assessments were performed in the supine (SUP) and lateral positions. The anesthesiologists responsible for the clinical evaluation were “blinded” to the bronchoscopic findings. While in the SUP position, the tube was “too deep” to permit visualization of the carina during tracheal bronchoscopy in 5 patients (2 RR, 3 PVC). In 17 of 21 (10 RR, 7 PVC), the bronchial cuff could not be visualized, although in 1 patient (RR group), the cuff was overinflated and bulged out to partially obstruct the right main bronchus orifice. Bronchial bronchoscopy showed 4 of 11 patients in the RR group in whom the left upper lobe orifice was occluded compared with 1 only in the PVC group. The PVC did not differ from the RR in cases in which the tube was “too far out.” However, they did differ in the incidence of the tube being pushed too far in 36% in the RR versus 10% in the PVC ( p 2 , PaCO 2 , tidal volume, and the peak airway pressures in all the patients in the PVC versus the RR DLT groups show no differences between the 2 groups. In the cases of malpositioned tubes, there were no statistical differences in PaO 2 between the right and left thoracotomies during two-lung ventilation (2LV) or one-lung ventilation (OLV) (520 ± 80 v 469 ± 56 mmHg and 167 ± 105 v 325 ± 94 mmHg, respectively). In the well-positioned tubes, the comparison between right and left thoracotomies showed no statistical differences in PaO 2 , (432 ± 114 v 464 ± 71 mmHg during 2LV and 182 ± 104 v 157 ± 94 mmHg during OLV, respectively). The results of this study show that, first, there is no significant difference in ventilation between the PVC and the RR tubes as measured by PaCO 2 , peak airway pressure, and tidal volume. Second, a similar incidence of malpositions was found in both groups in the supine and lateral decubitus positions except for a higher incidence of malposition in the RR group in which the bronchial carina was not visualized during bronchial bronchoscopy. Finally, the patients with malpositioned tubes had similar arterial oxygen saturation to patients with well-positioned tubes, whether the patients were undergoing right or left thoracotomies.