Objectives: This study aims to investigate the relationship between postural changes detected in ankylosing spondylitis (AS) vertebrae and clinicaland demographic characteristics of patients using a three-dimensional ultrasonic system.Patients and methods: A total of 44 AS patients (36 males, 8 females; mean age 39.1±9.8 years; range, 18 to 63 years) and 44 healthy volunteers(36 males, 8 females; mean age 38.4±9.4 years; range, 18 to 65 years) were included. Clinical, demographic, and laboratory data were recorded. Spinalposture analysis was performed using a Zebris CMS20 three-dimensional ultrasonic system.Results: Thoracic kyphosis angle was significantly higher in the AS group (45.4±12.8°) than in the control group (36.9±7.7°) (p=0.001). Lumbarlordosis angle was significantly lower in the AS group (20.7±10.6°) than in the control group (28±8.2°) (p=0.002). There were no significant differencesin total trunk inclination or sacral angles (p>0.05). Increase in thoracolumbar length at maximum spinal flexion and decrease in thoracolumbar lengthat maximum spinal extension were significantly lower in the AS group (p<0.05). A negative correlation was found between the thoracic kyphosisangle and length increase in spine flexure as well as chest expansion. Thoracic kyphosis angle had a statistically significantly positive correlation withage at onset of symptoms and tragus-wall distance (p<0.05).Conclusion: Significant changes were observed in spinal posture in patients with AS. In addition, significant correlations were found betweendynamic postural changes and spinal mobility. We think that postural evaluation and follow-up should be carried out with other parameters.Awareness of AS patients about postural changes should be increased, and proper exercise treatment should be applied.