Charts of 209 patients with ankylosing spondylitis were reviewed with regard to the occurrence of seronegative peripheral arthritis. Peripheral arthritis, excluding hip and shoulder involvement, occurred in 67 of the 209 patients (32%). No relationship was found between the age at onset of spondylitis and the onset of the peripheral arthritis. Among the 67 patients with peripheral arthritis, 19 (28%) had recurrent flares. X-ray findings similar to those of rheumatoid arthritis were seen in 22 patients (33%) with peripheral arthritis. Confusion about the cause of the peripheral arthritis frequently arose in patients who had the onset of the disease when the spondylitis was in remission.