Standard airway evaluation by oropharyngeal visualization, neck and jaw mobility tests and radiological assessment does not identify all patients at risk for failed intubation. Our objective in this study was to determine if acoustic reflection measurement of the upper airway can identify patients who are difficult to intubate and discriminate between them and the readily intubated population. We evaluated adults having a documented history of unexpected failed (n equals 14) or successful (n equals 14) endotracheal intubation using six combinations of body (sitting, supine) and neck (flexed, neutral, extended) position. Standard airway assessment revealed no differences between the study groups. Results of acoustic reflection analysis revealed multiple position dependent differences between the study groups. Absolute criteria to discriminate between the two groups were found with the patients laying in supine position with the neck extended. All patients who had been successfully intubated had pharyngeal volumes greater than 43.4 ml (mean +/- standard deviation, 55.0 +/- 8.8 ml), whereas pharyngeal volumes were less than 37.5 ml in all patients who had unexpected failed intubation (mean +/- standard deviation, 18.9 +/- 10.6 ml). This difference was significant (p < 0.05, Two Way ANOVA with post-hoc t testing). Acoustic reflection was 100% sensitive and 100% specific at distinguishing between patients who experienced unexpected failed intubation and those who had successful intubation. This potential screening test is fast (2 - 3 minutes), simple to perform and requires minimal patient cooperation.