Forty-five of 52 consecutive patients with chronic low back pain were screened for presence of HLA-B27 antigen one year after they were included in a rehabilitative program. Six (13.3%) were positive and, when re-examined radiographically, 2 had signs of ankylosing spondylitis. The proportion of antigen-positive individuals is similar to that found in a population study of healthy Swedish blood donors, and within the range of other populations of healthy controls. It is concluded that HLA-B27 is of limited diagnostic value as a screening test for ankylosing spondylitis in a patient group with chronic low back pain.