"B27 disease" is a new autoimmune disease that afflicts millions of people throughout the world. "B27 disease" occurs in individuals who have ankylosing spondylitis (AS) or preankylosing spondylitis and/or uveitis and are also positive for HLA-B27. Molecular mimicry between the bowel microbe Klebsiella and the HLA-B27 molecule, as well as the spinal collagens types I, III, and IV, indicates a pathological mechanism involving autoimmunity. Antibodies to Klebsiella microbes have been reported in AS patients from 18 different countries. Sera from patients with AS show complement-dependent cytopathic activity against sheep red cells coated with HLA-B27 peptide antigens. Diagnosis of B27 disease can lead to early treatment, involving low-starch diet, sulfasalazine, and immunosuppressive and biological agents so as to prevent the irreversible bony changes of established classical AS. The concept of B27 disease provides a new approach to the study and treatment of these disorders and needs to be evaluated in prospective studies by the world rheumatological community.