We conducted a prospective, blind study to determine whether a difficult endotracheal intubation could be predicted preoperatively by evaluation of one or more anatomic features of the head. In 471 adults presenting for elective surgery, the size of the tongue relative to the oral cavity was assessed according to the Mallampati classification (oropharyngeal class), and the distance between the chin and thyroid cartilage (thyromental distance) and the angle at full extension of the head (head extension) were measured. At laryngoscopy, the difficulty in visualizing the larynx was determined by a different observer. Assignment to oropharyngeal Class 3, a thyromental distance < or = 7 cm, and a head extension < or = 80 degrees, considered either alone or in various combinations, had low sensitivity and positive predictive values in identifying patients with airways that were difficult to intubate, but high specificity and negative predictive values. We conclude that these three tests are of little value in predicting difficult intubation in adults, although the likelihood of an easy endotracheal intubation is high when they yield negative results.