50 patients with ankylosing spondylitis without any peripheral arthritis are compared with 50 cases of ankylosing spondylitis complicated by arthritis of different kind. Radiological and clinical features were evaluated and assessed. HLA-DR4 typing was carried out. This antigen was found in 16% of 200 normal individuals, in 18% of all patients with ankylosing spondylitis confined to the spinal column, and in 54% of patients with only peripheral arthritis (wrist, finger, ankle, toe joints). The conclusion that HLA-DR4 represents nonspecific evidence for peripheral arthritis in patients with ankylosing spondylitis is discussed.