Ankylosing spondylitis (AS) is probably produced by repeated episodes of Klebsiella-reactive arthritis, usually in HLA-B27-positive individuals. This concept is based on immunological, microbiological and serological considerations. Immunological studies based on anti-B27 tissue typing sera and anti-B27 monoclonal antibodies indicate that HLA-B27 cross-reacts with antigens found in Klebsiella, Salmonella, Shigella and Yersinia micro-organisms. Salmonella, Shigella and Yersinia gut infections are associated with a reactive arthritis that occurs predominantly in HLA-B27-positive individuals. However, microbiological studies indicate that only Klebsiella, but not Salmonella, Shigella or Yersinia, can be isolated in faecal cultures obtained from AS patients. Furthermore, serological studies involving a number of different techniques demonstrate that only antibodies against Klebsiella, but not against Salmonella, Shigella or Yersinia, can be identified in active AS patients. The evidence is sufficiently extensive to warrant long-term studies involving Klebsiella reduction in the bowel flora of AS patients, in an attempt to reduce the severity and modify the development of the disease. It would appear that Klebsiella-reactive arthritis is the precursor stage occurring in the early and active phases of AS. Only future studies can determine whether this disease will remain a taxonomic curiosity or provide guidelines for therapeutic sequelae which will be of benefit to AS patients.