The HLA-B27 antigen was determined in 652 patients with low back pain which had lasted for more than 3 months. A clinical and roentgenological examination of the sacroiliac joints and the thoraco-lumbar spine was performed in all the patients. The control group consisted of 302 unrelated persons who did not show signs of low back pain. Antigen HLA-B27 was found in 276 of these 652 patients attending the ward for rheumatic diseases (42.4 per cent) and in 37 of the 302 unrelated persons in the control group (12.2 per cent). The difference is statistically highly significant (P less than 0.001). Ankylosing spondylitis was found in 128 out of the 276 patients with low back pain and antigen HLA-B27. This demonstrates the importance of this antigen in the differential diagnosis of low back pain.