Fifteen patients with ankylosing spondylitis who had developed a severe flexion deformity of the cervical spine which restricted their field of vision to their feet, were treated by an extension osteotomy at the C7/T1 level. The operation was performed under general anaesthesia with the patient in the prone position and wearing a halo-jacket. Three had internal fixation using a Luque rectangle and wiring. Their mean age was 48 years. Before operation the mean cervical kyphosis was 23 degrees; this was corrected to a mean of 31 degrees of lordosis, a mean correction of 54 degrees. All the patients were able to see straight ahead. One patient with normal neurology soon after operation became quadriparetic after one week; two others had unilateral palsy of the C8 root, which improved. There was subluxation at the site of osteotomy in four patients, and two of them developed a pseudarthrosis which required an anterior fusion.