Fourteen patients with ankylosing spondylitis had an extension osteotomy for severe flexion deformity of the spine. The Smith-Petersen technique was modified by using a compression device which allows a slow, finely controlled closure of the osteotomy, and provides rigid internal fixation. There were no serious neurological complications. All the patients were able to see straight ahead after operation, and all had solid fusion at nine months, having maintained good correction.