An epidemiological survey of ankylosing spondylitis (AS) was performed in Tromsø, Northern Norway. Altogether 27 cases of AS were found, and the symptoms and signs of these subjects were compared to those of persons with back pain but not suffering from AS (NON-AS). The symptoms which differentiated best between AS and NON-AS were: back pain which awakened the subject at night and made him leave the bed, back pain not relieved by lying down but improved by exercise, back pain of 3 months duration or more and morning stiffness lasting 0.5 hours or more. Reduced lateral mobility of the lumbar spine, a total spinal extension of 20 degrees or less and a total spinal flexion of 40 degrees or less were signs of acceptable value for the diagnosis of AS. Clinical criteria for the diagnosis of AS, to be used both in the clinical management of patients with back complaints and in population surveys of AS are suggested.