BACKGROUND: Using the intubation difficulty scale (IDS), we sought to confirm that obese patients are more difficult to intubate than lean patients. We assessed classical bedside tests and included neck circumference. METHODS: We prospectively compared the incidence of difficult tracheal intubation in 70 obese [body mass index (BMI) > or = 30 kg/m(2)] and 61 lean patients (BMI < 30 kg/m(2)). The IDS scores, categorized as difficult intubation (IDS > 5) or not (IDS < or = 5), and the patient data, were compared between lean and obese patients. Preoperative measurements [BMI, neck circumference (at the level of the thyroid cartilage), width of mouth opening, sternomental distance, and thyromental distance], medical history of obstructive sleep apnea syndrome, and several scores (Mallampati, Wilson, El Ganzouri) were recorded. The view during direct laryngoscopy was graded, and the IDS was recorded. We then compared patients with IDS < or = 5 and > 5, concerning each item. RESULTS: The results indicate that difficult tracheal intubation is more frequent in obese than in lean patients (14.3% vs 3%; P = 0.03). In the patients with IDS > 5, thyromental distance, BMI, large neck circumference, and higher Mallampati score were the only predictors of potential intubation problems. CONCLUSION: We found that problematic intubation was associated with thyromental distance, increasing neck circumference, BMI, and a Mallampati score of > or = 3. Neck circumference should be assessed preoperatively to predict difficult intubation.