To determine the rate of unrecognized endotracheal tube misplacement when performed by emergency medical services (EMS) personnel in a mixed urban and rural setting. The authors conducted a prospective, observational analysis of out-of-hospital endotracheal intubations (EIs) performed by EMS personnel serving a mixed urban, suburban, and rural population. From July 1, 1998, to August 30, 1999, emergency physicians assessed and recorded the position of out-of-hospital EIs using auscultation, direct laryngoscopy, infrared CO(2) detectors, esophageal detector devices, and chest x-ray. The state EMS database also was reviewed to determine the number of EIs involving patients transported to the authors' medical center and paramedic assessment of success for these encounters. A total of 167 out-of-hospital EIs were recorded, of which 136 (81%) were deemed successful by EMS personnel. Observational forms were completed for 109 of the 136 patients who arrived intubated to the emergency department. Of the studied patients, 12% (13 of 109) were found to have misplaced endotracheal tubes. For the patients with unrecognized improperly placed tubes, 9% (10 of 109) were in the esophagus, 2% (2 of 109) were in the right main stem, and 1% (1 of 109) were above the cords. Paramedics serving urban and suburban areas did not perform significantly better (p < 0.05) than intermediate-level providers serving areas that are more rural. The incidence of unrecognized misplacement of endotracheal tubes by EMS personnel may be higher than most previous studies, making regular EMS evaluation and the out-of-hospital use of devices to confirm placement imperative. The authors were unable to show a difference in misplacement rates based on provider experience or level of training.