Purpose Although manual in-line stabilization (MILS) is commonly used during endotracheal intubation in patients with either known or suspected cervical spine instability, the effect of MILS on orotracheal intubation is poorly documented. This study evaluated the rate of failed tracheal intubation in a fixed time interval with MILS. Methods Two hundred elective surgical patients were randomized into two groups. In the MILS group, the patient's head was stabilized in a neutral position by grasping the patient's mastoid processes to minimize any head movement during tracheal intubation. In the control group, the patient's head rested in an optimal position for tracheal intubation. A 30-sec period was allowed to complete tracheal intubation with a #3 Macintosh laryngoscope blade. The primary endpoint was the rate of failed tracheal intubation at 30 sec. Secondary endpoints included tracheal intubation time and the Cormack & Lehane grade of laryngoscopy.Results Patient characteristics were similar with respect to demographic data and risk factors for difficult tracheal intubation. The rate of failed tracheal intubation at 30 sec was 50% (47/94) in the MILS group compared to 5.7% (6/ 105) in the control group (P \ 0.0001). Laryngoscopic grades 3 and 4 were more frequently observed in the MILS group. Mean times for successful tracheal intubation were 15.8 ± 8.5 sec and 8.7 ± 4.6 sec for the MILS and control groups, respectively (mean difference 7.1, CI 95% 5.0-9.3, P \ 0.0001). All patients who failed tracheal intubation in the MILS group were successfully intubated when MILS was removed. Conclusion In patients with otherwise normal airways, MILS increases the tracheal intubation failure rate at 30 sec and worsens laryngeal visualization during direct laryngoscopy. RésuméObjectif Bien que la stabilisation manuelle en ligne (SMEL) soit fre´quemment utilise´e pendant l'intubation endotrache´ale chez des patients pre´sentant une instabilited e la colonne cervicale connue ou suspecte´e, l'effet de la SMEL sur l'intubation orotrache´ale est tre`s peu documente´e. Cette e´tude a e´value´le taux d'e´chec de l'intubation trache´ale dans un intervalle de temps fixe avec une SMEL. Méthode Deux cents patients devant subir des chirurgies non urgentes ont e´te´randomise´s en deux groupes. Dans le groupe SMEL, la teˆte du patient a e´te´stabilise´e dans une