The upper airway abnormalities predisposing to difficult tracheal intubation may also predispose to obstructive sleep apnoea (OSA). The potential association is important as both conditions increase perioperative risk and patients with a trachea that is difficult to intubate may need assessment for OSA. We determined if patients with difficult intubation are at greater risk of OSA and, if so, whether or not they have characteristic clinical or radiographic upper airway changes. We studied 15 patients in whom the trachea was difficult to intubate and 15 control patients. Each was evaluated clinically (Mallampati score, thyromental distance, neck circumference, head extension), polysomnographically (apnoea-hypoponea index (AHI)) and radiographically (lateral cephalometry). AHI was greater in the difficult intubation group (mean 28.4 (SD 31.7)) compared with controls (5.9 (8.9)) (P < 0.02); eight of 15 patients in the difficult intubation group and two of 15 in the control group had an AHI > 10 (P < 0.03). Difficult intubation, but not OSA, was associated (P < 0.05) with a smaller thyromental distance and mandibular length, and greater soft palate length. Both difficult intubation and OSA were associated (P < 0.05) with a greater Mallampati score, anterior mandibular depth, and smaller mandibular and cervical angles. OSA, but not difficult intubation, was associated (P < 0.05) with increased neck circumference, tongue area and craniocervical angle, and decreased head extension, mandibular ramus length and atlantooccipital distance. We conclude that difficult intubation and OSA are related significantly. They share anatomical features which act to reduce the skeletal confines of the tongue. Patients with OSA may compensate, when awake, by increasing craniocervical angulation, which increases the space between the mandible and cervical spine and elongates the tongue and soft tissues of the neck.