The authors present a case of endotracheal tube (ETT) obstruction during stereotactic computer-assisted volumetric resection of an enlarging left thalamic mass in a prone patient. The head was fixed in a stereotactic COMPASS head frame and the patient positioned on transverse hip and shoulder rolls. The prone position was achieved with attention given to ensure an adequate distance between the sternum and mentum. At the time of dural incision, the patient's peak airway pressure and end-tidal CO2 tension increased markedly whereas the O2 saturation remained stable at 99%. It immediately became clear that the patient's torso had moved cephalad resulting in ETT obstruction, despite the continued integrity of head fixation. We emphasize the need for proper and safe patient positioning, the placement of a bite block between the molar teeth, and particularly in obese patients, the need to tape the patient's torso securely to the operating room table. In addition, it may be necessary to place the operating room table in reverse Trendelenberg position to prevent cephalad movement of the torso. This case represents an unusual mechanism of ETT obstruction, as normally the latter is caused by flexion of the head rather than by cephalad movement of the torso.