Objective: In the present paper authors evaluated, on supine lateral head films, the correlation of Upper Airway Space (UAS), posterior maxillary structures (i.e., palatal morphology), maxillo-mandibular and hyoid position with different severity of OSA evaluated on polysomnography test. Materials and Methods: One hundred patients (mean age 51.4 years old; 92 man and 8 women) with OSA were enrolled in the present study. An Epworth questionnaire and a complete overnight Polysomnography (PSG) were done. Head films were taken with the patient in supine position fixed in a cephalostat, in centric occlusion, with adequate visualization of reference structures. Results: The cephalometric evaluation showed how patients presented a reduction of Upper airway space, a post inclination of the mandible, an increase of the ANB Angle and an increase of palatal length, palatal height and palatal angle. At Spearman test a great correlation between IAS and OSA data, a direct correlation between the palatal length and thickness with IAS and an inverse correlation between the mandibular post inclination and backward position and IAS were evaluated. Conclusion: In the present paper authors showed how PSG (AHI, SO2 and Nadir) data was correlated above all to IAS. The modification of IAS was related to the mandibular vertical and sagittal position and to palatal length and thickness.