This prospective, blinded, observational, efficacy study is one of the first to evaluate ultrasound in detecting esophageal intubation, a significant source of morbidity and mortality. We utilized a convenience sample of patients undergoing elective surgery during July 2004 in an urban teaching hospital. Trained Emergency Physician sonographers performed transtracheal ultrasounds of intubations to identify esophageal intubation. In 35 of the 40 patients enrolled, there was intubation of the trachea, whereas esophageal intubation occurred in five patients. Sonographers correctly identified all five esophageal intubations, for a sensitivity of 100% (95% confidence interval [CI] 48-100). Ultrasound correctly identified 34 of 35 tracheal intubations and misidentified one resulting in a specificity of 97% (95% CI 90-100). It seems that transtracheal ultrasound may be an efficacious adjunct for detecting esophageal intubation.