To clarify the relationship between head posture and pharyngeal airway space (PAS), the cephalometric parameters at different head postures were examined. Twelve normal Japanese adults (6 males and 6 females) were examined. Lateral cephalometric radiographs were taken at five different head postures in each person. All radiographs were traced, and the measurements were analyzed statistically. PAS was significantly increased by forward inclination of the cervical spine. The most significant correlation was found between the change in CVT/NSL (cranio-cervical inclination in the second and fourth vertebrae) and the change in PAS-TP (the minimal pharyngeal airway space) (r(2)=0.79 in males, r(2)=0.67 in females). The mean CVT/NSL when the head was in the natural (neutral) position was 100.9 degrees in males and 103.5 degrees in females. Linear regression analysis revealed DeltaPAS (mm)=0.37DeltaCVT/NSL (degree) (r(2)=0.79, p<0.0001) in males, and DeltaPAS (mm)=0.33DeltaCVT/NSL (degree) (r(2)=0.51, p<0.0001) in females. The correlation equations were obtained as follows: the corrected PAS (mm)=the actual PAS (mm)+0.37[100.9-the actual NSL/CVT (degree)] in males, and the corrected PAS (mm)=the actual PAS (mm)+0.33[103.5-the actual NSL/CVT (degree)] in females. These results will contribute to obtaining an accurate assessment of the PAS that should be corrected by the cranio-cervical inclination.