We compared diaphragmatic motion between ankylosing spondylitis (AS) patients and controls, as assessed by the ultrasonographic method. We included 33 consecutive AS patients (19 males, 14 females) followed up at our center and 14 apparently healthy controls (8 males, 6 females) into our study. AS patients fulfilled the modified New York classification criteria for AS. Patients' demographic and clinical data, functional parameters, and radiographic findings were recorded down. By evaluating the motion of right and left diaphragm during deep expirium and inspirium, the mean diaphragmatic motion was determined by ultrasonography. Diaphragmatic motion in AS patients was less than in controls, but the difference was not significant (68.9 +/- A 17 mm vs. 77.8 +/- A 22.4 mm, P = 0.14). Diaphragmatic motion in AS patients who were active according to BASDAI score (> 4) was not different from inactive patients (70.4 +/- A 20.5 vs. 67.5 +/- A 13.5, P > 0.05). The mean diaphragmatic motion had a positive correlation with occiput-to-wall distance (r = 0.35, P = 0.048); and negative correlations with cervical rotation (r = -0.45, P = 0.01) and modified Schober test (r = -0.34, P = 0.05) in AS patients. We did not detect any association of mean diaphragmatic motion with thoracic expansion on deep expiration. Diaphragmatic motion in AS does not differ significantly from the control group. Factors like disease activation, chest expansion, and the severity of radiographic findings do not affect diaphragmatic motion. There is no compensatory increase in diaphragmatic motion in AS.