Although intubation of emergency patients in the field is a routine measure, endotracheal tube misplacement remains a serious problem. Using radiologic criteria, the frequency of undetected endobronchial intubation by physicians was determined retrospectively in 100 (78 traumatized) field-intubated adult patients (72 men and 28 women; age, 18 to 90 years; mean age, 39.1 years) consecutively admitted to the University Hospital of Tuebingen, Tuebingen, Federal Republic of Germany, between January 1987 and February 1988. Position of tube tip relative to carina was evaluated on anteroposterior chest radiographs made on admission. Inadvertent endobronchial intubation was not recognized by the physician and the admitting anesthesiologist in 7% of the reviewed cases, and endotracheal positioning of the tube tip near the carina (2 or less cm) occurred in another 13%. While unilateral intubation is not immediately catastrophic, the resulting systemic hypoxemia and hypercapnia are aggravated by potential accompanying injury (eg, lung contusion, hematothorax, pneumothorax, shock, or cerebrocranial trauma), which can lead to secondary damage (eg, acute respiratory insufficiency, ischemic brain damage). Evaluation of the depth of tube insertion with the aid of common clinical techniques is particularly unreliable in the case of thoracic trauma, aspiration, or previously existing pulmonary disease. Suggested measures for prevention of endobronchial intubation are improved and intensified training of emergency staff to increase awareness of and prevent the catastrophic effects of endobronchial malposition of the tube tip, tube shortening before intubation, assessment of insertion depth by checking length scale on the tube, and avoidance of patient head and neck movement.