Title Size of atlas and axis in adults with hyphofosphatemic rickets – a comparative study. Introduction. Deviations from normal anatomy of the cervical column are frequent in some pathologic conditions (cleft-patients, OSA-patients) and in severe malocclusions. Hypophosphatemic rickets (HR) are rare diseases characterized by deficient mineralization of the bones due to abnormal renal wasting of phosphate. Cephalometric studies of HR-patients have shown that the osseous structures of cartilaginous, as well as of intramembranous origin, are affected in the cranium. The cervical vertebrae are bony structures of cartilaginous origin. Aim It is the aim to describe the size of the atlas and axis in adults with HR in comparison with healthy controls. Material Standardized profile radiographs of 36 patients with hypophosphatemic rickets (24 females, 12 males), aged18 – 74 yrs, and 49 controls (26 females, 23 males), aged 18 – 79 yrs. Method The outcome cephalometric variables were: a) the height of the anterior tubercle of the atlas, b) the height of the dorsal arch of atlas, and c) of axis, d) the length of atlas, and e) the height of dens. The comparison between HR-patients and controls were performed by linear regression analyses adjusting for age and gender. Results The comparison between HR-patients and controls showed significant (p≤0.05) differences in the following outcome variables (mean HR-patients (SD), mean controls (SD)): a) height of the anterior tubercle of atlas 12.85 (1.89), 11.23 (2.38); b) height of the posterior arch of atlas 11.96 (1.46), 11.19 (1.71); c) height of the posterior arch of axis 20.78 (3.60), 17.42 (8.65); d) length of atlas 54.37 (3.34), 52.84 (4.43). Conclusion: In HR-patients, the horizontal and vertical dimensions of axis and the height of the posterior arch of axis were large in comparison with healthy controls.