Signs of ankylosing spondylitis were sought in 11 patients suffering from Behcet's syndrome and followed up in different departments of the Clermont-Ferrand teaching hospital group. Three patients, described in detail, had definite ankylosing spondylitis and 2 had phenotype B27. This prevalence would be greater than that predicted on the basis of data in the literature. This combination is rare but it is felt to be legitimate to seek the other disease when one of these two conditions is diagnosed.