In 95 Swiss patients with classical ankylosing spondylitis (AS) the tissue antigen HLA-B27 was present in 92.6%, compared with 7.7% in healthy Swiss blood donors. Assuming the prevalence of ankylosing spondylitis in Switzerland to be 1.9 promille, the chance of a Swiss carrier of HLA-B27 to develop a classical form of AS would be only some 2.2%. For diagnostic purposes, HLA typing thus seems to be of very little value, as among the 462 000 Swiss carriers of HLA-B27 there seem to exist no more than 10 800 classical cases with clinically manifest AS. Absence of HLA-B 27 does not exclude ankylosing spondylitis, as 7.4% of the classical cases are HLA-B27-negative. However, the crudely calculated risk to develop AS is 160 times smaller compared to a carrier HLA-B27. Corner stone of the diagnosis therefore remains careful case history and radiological features of a bilateral sacroileitis of at least grade II.