The first 2000 incidents reported to the Australian Incident Monitoring Study were analysed with respect to problems with the endotracheal tube; 189 (9%) were reported. The most common problem was endobronchial intubation which accounted for 42% of these 189 reports; endobronchial intubation was the most common cause of arterial desaturation in the 2000 incidents. Obstructions and oesophageal intubation each accounted for 18% of the 189 problems with tubes. The remainder was made up of disconnections and leaks (7% each), misplacements other than endobronchial or oesophageal (4%), inappropriate choice of tube (3%), cuff herniation (1%), failure to deflate the cuff and foreign body in the tube (0.5% each). The pulse oximeter and capnograph first detected 58% of these incidents; a further 25% were detected clinically. The pulse oximeter is the "front-line" monitor for endobronchial intubation, and the capnograph the "front-line" monitor for oesophageal intubation, disconnection and obstruction. Recommendations are made for how to prevent problems and how to determine the nature of those that do occur.