The anatomical factors in difficult direct laryngoscopy have been assessed. Thirteen patients in whom direct laryngoscopy proved difficult and thirteen control patients, all with teeth, had lateral, postero-anterior and submento-vertical radiographs taken. Several measurements were made and compared. The most important factor determining the ease of examination was the posterior depth of the mandible. An increase in this measurement was thought to hinder displacement of the soft tissues by the laryngoscope blade. Other factors of importance were: an increase in the anterior depth of the mandible; a reduction in the distance between the occiput and the spinous process of C1, and to a lesser extent the C1-C2 interspinous gap; and reduced mobility of the mandible associated with temporo-mandibular joint arthritis or trismus.