Testing for the histocompatibility antigen HLA-B27 has been suggested as a valuable diagnostic aid in patients with ankylosing spondylitis and associated spondyloarthropathies, and B27 typing is becoming more readily available commercially at a number of clinical pathology laboratories. A current evaluation of B27 typing as a diagnostic test is presented, together with some guidelines for the clinical application of this test in the diagnosis of certain rheumatic diseases.