BACKGROUND: Tracheal intubation may be difficult in many patients with sleep apnea syndrome because of anatomical abnormalities in their upper airway. METHODS: For 30 patients with obstructive sleep apnea syndrome, we evaluated two classifications; Samsoon-Young and Cormack-Lehane, and five airway examinations; interincisors' distance on mouth opening (DI), angle on cervico-occipital extension (ACO), thyromental distance (TD), and existence of edentulous with atrophic mandible and prominent maxillary incisor. RESULTS: 20 patients (67%) belonged to class III or IV according to the Samsoon-Young classification and 22 patients (73%) to grade III or IV according to the Cormack-Lehane classification. The incidence rate for DI less than 40 mm, ACO less than 160o, TD less than 60 mm, atrophic mandible and prominent maxillary incisor were 30%, 37%, 50%, 13% and 27% respectively. CONCLUSIONS: According to our results, we concluded that most patients with obstructive sleep apnea syndrome are difficult for anesthesiologists to intubate. Therefore, we suggest that anesthesiologists must prepare variable methods for ventilation if there is any suspicion of difficult intubation in those patients.