Atlanto-axial subluxation and dislocation in rheumatoid arthritis is a multidirectional abnormality. The most common subluxations take place anteriorly, vertically or laterally. Posterior atlanto-axial subluxations are rare. The distance between a line tangential to the anterior aspect of the axis (C2) and the most posterior aspect of the anterior arch of the atlas (C1) could be used to measure the displacement in posterior atlanto-axial subluxation. Routine measurement of this distance could give us a better idea of the frequency of this type of affection in rheumatoid arthritis of the cervical spine. Serial measurements might be used to estimate the progression of the disease.